A picture says a thousand words. It is also next to impossible to sell art without some sort of image (although I remember in the earlier days of ebay, people trying to do just that. They were ususally the ones with $50 000 000 opening bids.) There are a few parts to obtaining beautiful images that will sell your work.
Your Artwork: You have a lot of competition out there. It is to your greatest advantage that you put up really beautiful work. If it looks a little unfinished to you, it will likely look a little unfinished to your potential buyers. Take the time to finish it up right (and also neaten edges, attaching hanging hardware if necessary - all this can be mentioned in your listing and will attract customers)
Using a Camera: If you are using a camera to take a picture you need to ensure that your art is properly lit and in focus. Never ever assume your customer is as visionary as you are - they are coming to purchase artwork with a "What You See Is What You Get" point of view. You will never totally convey what an original piece looks like in real life, but you can really get close with today's technology.
I usually wait till I have a clear, sunny day and take my work out and position it so that there are no glare marks from the sun hitting it. This may take a bit of fiddling but is worth it. Pure daylight is the best way for you to capture the rich hues and depth of your artwork.
If you have a piece that you are going to sell framed, under glass I recommend that you capture your image without the glass to avoid glare and refraction. You can take a second picture to show how nice it is framed and matted and to give the customer an idea of what the purchased work will arrive looking like, but you do need a shot of your work as is, without frame.
If you cannot use daylight, choose ample warm lighting (no cold flourescent lighting) and position the lighting so that you have no glare or shadows falling across your work.
Try to fill up as much of the frame with your image but allow for a little room to crop close. I really recommend that you purchase a digital camera if you do not have one - print film will require scanning later on and you will not get as detailed an image.
Using a Scanner: This is a great way to capture high resolution, well lit and colour balanced image files. It is especially great for non textured paintings and drawings. Some will be small enough to just pop on the scanner bed and go. Before pressing "scan" read the digital file info below - best to start big and go smaller than try to go big from small.
If your image is too big for the scanner bed and you have Adobe Photoshop (not sure what other programs can do this) you can scan your piece in sections (you will have a bunch of separate files then) Make sure they are all open in Photoshop and go to "file" select "automate" and select "photomerge" - follow the prompts and you will end up with a large file that has meshed all the components of your image into one. Just make sure you scan all the parts of your artwork - overlapping a little for each part allows the end result to be seamless.
Digital Master Files: no matter how you obtain your image, you will need to have it in a digital format. I recommend a master copy that is at least 300 dpi. You camera or scanner should have an option to set this and what file type you prefer.
If you can, use a non compression file (ie: TIFF) or save in your editing program as a non compression file as soon as possible so you don't lose the digital information like you would in a compressed format (ie: JPEG) Keep the the TIFF file untouched (except cropped as close to the outer edges as possible) for future reference. Keep a copy burned to disc for your records.
You may one day need this image for a print magazine article, for a licensing project or to compile a book - if you sell your piece and it ships overseas, it would be quite complicated to get the original back to take a quality master copy. Start this habit now and it will become second nature to you for all of your artwork.
Digital Files for Online Selling: Next, you will want to make a duplicate copy of your master and rework it for selling online. Make sure the image is cropped as close to the edges as possible.
In your photo editing program, you will want bring your image as close to real life as possible (this may include adjusting the colour balance if your yellows are a bit underwhelming in the photo, cropping, removing a dust mark) Do not enhance your image or change a detail that is not changed in real life - you can do that later if you are selling the image to license a product, but you cannot do that to sell the original. It needs to be "What You See Is What You Get".
Watermarks: You may want to add a watermark, this is the "© Your Name Year Made" - there are no set rules to this but some things to keep in mind. People theoretically may want to take your image without your authorization - if you post it online, you take that risk.
I feel that by not having too large an image file (you shouldn't for a number of reasons but this is one of them) will prevent someone taking your image to profit from it. Other uses like someone uploading it as a backdrop on their phone just chalk it up as free advertising and that someone loves your work so much, they wanted to upload it to their phone.
If you add a watermark, remember that the more of your image it takes up, the less "What You See Is What You Get" can happen. You do not need to use 72 pt font hot pink letters to convey that you are the copyright owner of your work. (you automatically are even when you sell your piece - but that is another article) Choose a discrete place that is essential to your image (so it can't be cropped off) and do a small watermark.
My favorite way to do a watermark is go into Photoshop, type it in with white and then slide the opacity down to about 15% - that makes it see through and is very subtle and undistracting. I then flatten the layers and proceed with my watermarked image.
If you are stumped on how to do the © sign, have no fear - you can always just highlight that one and cut and paste into your image document OR you can hold down the Alt button and type 0169 - that will give you that little © symbol.
Online File Size: Next you want to reduce the size of your image. You want it small enough so that it is uploadable to sites on the web and isn't easy to steal and blow up to poster size with clear crisp details. The usual is 72 dpi. Next you will want to look at the dimensions. If it is an ACEO (2.5"x3.5") you do not need to reduce the size. If you have photomerged a 2' x 3' painting, that will still be a massive file. You can take down the dimensions by reducing the pixel dimensions (I usually just reduce it so the file is under 500K and is 72 dpi).
One thing to keep in mind is that some sites have specific dimensions that they accept - it would benefit you to find out what they prefer and then get as close to the upper limit as possible. (ie. etsy likes 1000 pixels by 1000 pixels (72 dpi), ebay has similar limits.)
Finally, save in a compressed format, JPEG is pretty standard. This image can now be uploaded to a number of venues and start selling your artwork.
By following these steps, you will now have quality images that you can not only use to sell the original, you can use in the future for promotion, a portfolio or licensing options - you may make a lot of money from a quality image from your original.