Call for Artists, Making Money at Juried Art Fairs, Craft Shows and Festivals
They need to be photographed with a high enough resolution camera (height and width in pixels) so that the file can be printed.
Rule of thumb is 300 pixels per inch. That translates to 2400x3000 pixels for a high quality 8x10 print. If they are to be printed on canvas, you can get away with 180 or 200 pixels per inch so a smaller file can be used.
The photograph has to be taken with the camera on a tripod and good lighting. The camera needs to be squared to the painting so there's no distortion in the print, which is called a reproduction because the painting is the original.
As Larry said, you need to start with a good photograph. Make sure your photo is cropped to show just the artwork; but try not to have to crop too much. A photograph that is already zoomed in tight to your artwork, then just trimmed a little around the edges works best. Then when choosing printers, you have a huge amount of options. There are some discount printers such as CanvasDiscount.com and Canvas on Demand that run specials all the time, and can print you out an 16" x 20" gallery wrapped canvas for about $ 25 - 30. They are pretty simple to use, and do a decent job. But pay extra to get the color correction service so your print colors match the photo best. I also read a good review of a company called CanvasPop. They are offering a deal on 16" x 24" canvas print for $48 now. Of course, there are higher quality printers, but most will charge a lot more. I use two companies - FinerWorks and Giclee Today - for my prints, and am quite pleased with the quality. FinerWorks is based in Texas and Giclee Today is in Pennsylvania. Both are online. Giclee Today has a faster turn around time for me, and I think their prices are more reasonable, but they sometimes require a little more technical ability on the part of the person submitting their artwork, ie you need to know how to set borders with Photoshop, how to set the pixels size on your files, etc. With FinerWorks, you can just upload your photo and tell them how you want it to look.
Both of the responses are right on. I took classes in how to photograph my paintings, but essentially, everything Larry said is going to be right (he's a professional.)
Simply, get an inexpensive tripod with a remote clicker. Your smartphone takes excellent photos and you likely have an editing program right on your phone. But you need the tripod to hold it steady and straight, and you need the clicker so you don't have to touch the phone.
Use natural daylight, but no direct sun.
I have used Finerworks in San Antonio for the last 8 years. They are very reliable and offer LOTS of options for your product. You can even sell with them. I trust them enough that I drop-ship to clients directly. As Sonja said, they aren't the fastest, but I've never waited more than 2 weeks, and if there has ever been something they questioned, they would email me before printing. Paying a few dollars more has returns in the quality of presentation, too. I've been able to paint on prints from Finerworks, and NOT from some of the cheaper alternatives. (Sometimes a client wants some color changes and the quality of the print and canvas, as well as finish, is important.)
Your biggest challenge is going to be "shine" or reflection, which is why daylight but not direct sun.
I've been photographing my own paintings for almost 20 years. It takes some practice, but there are online DIY videos, too.
Reproductions or copies. Not prints.