You need two lights with 12" to 20" reflectors that will have the same output, they can be hot lights or strobes. Mount the picture on a bulletin board and hold it flat with tack flanges like in the example below. Don't use a glass overlay because the reflections will kill the contrast. Arrange the lights to either side so they are the same distance from the picture and hit it at 45°... then adjust them so the center of the beams cross and strike the farthest edges of the picture and then feather them slighly further so the hot spot is off the picture. That will make the light about as even as you can make it unless you add diffusers. Don't put the lights really close, 5' or 6' is fine, 2' will give you problems with falloff. You can do all this with the bulletin board mounted on a wall and the camera on a tripod.
You have to get your camera square to the picture or you'll get a trapezoid instead of a rectangle. The easy way to do it is to use a piece of mirror. Before you mount a picture, mount the mirror (no frame) right in where the center of the picture will be... be precise, you can mark the outline of the pictures on the bulletin board to make it easy to get each one in the right place. Place your camera on a tripod and position it at the distance which wil fill the frame with the picture (leave a border but only a tiny one). Now the fun part... through trial and error move the camera and tripod until you can focus your lens in the very center of the mirror in the center of your frame. It's gonna suck because you have to do lots of little tiny movements of the tripod and the compenstae with the camera angle which is gonna make you have move the tripod again to re-center the image of your lens. If you do it a lot you get good at it but the first few times are a pain.
Anyway, when your lens is focused in the center of your mirror in the center of the camera frame, you're square. DON'T MOVE ANYTHING AFTER THAT... or you'll have to do it again. (the example image looks trapezoidal by the way but it's square on; the paper was actually cut that way)
Last, if the images are color, put a grey card in the picture right next to the image for every shot so you have a reference to set WB. I would shoot RAW so WB can be tweaked later.
If reflections from texture in the photo paper is a problem (it will look like inexplicably low contrast), you will need to polarize the light and use a lens polarizer to filter it out. You can get Roscoe polarizing film for your lights from a theatrical lighting supply but it's expensive.
Good luck,
Chip