How to photograph your own paintings:
First, DON'T. Find a professional photographer to do it for you.
Okay, you can't get a professional. Try to get your hands on a very large copy stand with four lights.
Okay, you can't do that either. That's why you're on line Googling "How to photograph paintings."
Here are my “Photo Tips for the Broke” on photographing paintings. This is assuming you can spend no money at all on equipment.
1. Set your camera on the highest resolution possible. Seems like a no-brainer, but sometimes you find you’ve accidentally reset it to a lower res. If your camera isn't good enough, borrow one.
(If you can only get a film camera, take the film to Target or whatever and they will put the images on a disc for you. It's much cheaper than having prints made.)
2. Download GIMP (GNU Image Manipulation Program). It’s free, and it works pretty much like Photoshop. Most things you can fix by just cropping and brightness/contrast adjustment. You can also level it if it’s crooked. The “white balance” function usually fixes mine right up.
3. Hang the picture if possible. If not, set it on a box and prop that up against the wall. Don't worry about the background because you're going to crop it.
4. Use a level to make sure the picture is straight. You can straighten it in GIMP if it's not, but it's easier to level it in the first place.
5. Never, ever, ever use regular incandescent lighting. Sunlight is free. The best light would be out of doors on a sunny day but not in direct light. One good tip I read was to make a tent out of white sheets and shoot inside that in direct sunlight, but I haven't tried that yet.
Indoors, you want two 100-watt full spectrum lights at 45 degree angles from the painting. If you use artificial lights, use something to diffuse the light, like tracing paper.
6. Use a tripod if you have one. If not, stack up boxes or whatever until the camera is at the right height. Use the timer. If you're using natural light, set the ISO to 800.
7. Always crop the picture so it only shows your painting. You can do a separate shot of it hanging on a wall in a room to show how it looks hung, but right now we're just working on getting the most accurate possible photo of your painting. Nobody wants to see the background.
8. Save the best version as a .png or .gif file. You can make .jpg files of it if that's what you need to upload to a site, but .jpgs degrade every time they're saved. If you save the best version as a .png or .gif, it won't be as much of a problem if you accidentally resave it.