I know you can change a .art image into a .jpeg easily, but is there a way to turn a .jpeg into a .art file?
what kind of computer?
try download.com they might have something if you search.
or take a stab at it & search for say, art2jpg 
THats usually how they name programs that convert.
JPEG is a very common fie format, and just about every program that handles fotos os bitmaps should be abe to save as jpeg.
.ART, however is decidedly odd. According to this list it could be either a “Xara Studio drawing”, a “Canon Crayola art file”, generic “Clip Art”, “Ray Tracer file”, “First Publisher Raster graphic” or “AOL Image file compressed using the Johson-Grace compression algorithm”. The best guess would probably be to find the apropriate program, and see if it can save as JPEG. If that fails, maybe you can save as .bmp, .gif, .png or any other lossless image format, and try to open in another program that handles JPEG. And if that fails, try to use some OS-specific way of copying the image to another program (ie copy-paste, or screen capture). Most windows programs can handle copy-paste of images straight forward.
Another advice (assuming you use Windows) is to get hold of ACDsee, an excellent small utility to view/convert/edit images. Nowhere near as fully-featured as Photoshop, but FAST and CHEAP ($40, or for free if you don’t mind looking at their adds.) http://www.acdsystems.com
I may be able to do that in Photoshop. Unfortunately, I’m at my day job right now, and the machine with Photoshop is at my studio so I can’t be sure. I will be there later on this evening.
If you want to, e-mail it to me I’ll see what I can do then.
I’m assuming the .art format you want is the AOL brand, not one of the others tc mentioned. Are you wanting to convert to .art to post pictures on an AOL website? If so, I think the AOL pp2 program will automatically convert .jpg files to their format.
If it’s for some other use, you may be out of luck, because I don’t think most graphics programs can read the .art format. It’s a proprietary AOL program, and you know how they are. I tried PhotoShop 5.5, and .art was not one of the “save as” or “export” choices. It’s easily possible that I just don’t know how to do it, though.
This site seems to suggest you might be able to convert through AOL and save the file: