From: Daniel Piechnick
Hi Bill. I hope you have a fast computer for this :) Scan the paintings in bit by bit, but leave an overlap at the edge of each piece. In Photoshop, make a new image, and make it huge (bigger than all your scans put together). Open all your scans, and drag them into the big image. Zoom out (Ctrl -). With the Move Tool, drag the images around, and line them up. Zoom in (Ctrl +) and get it perfect. If you need to rotate any of the images, press Ctrl+T (This bit won't be easy). Once everything's lined up, you can save your image. (Remember to save it as a Photoshop PSD file.) All this could be very slow, given that you'd want to be doing all this at mega-high resolution.