I use JASC Paint Shop Pro 9. The key is to set up a grid.

My large scans on my website are 240x240 pixels. In PSP I set up a blank image 2400x1440 pixels. I then set up a grid of 240x240 boxes on the image. At this point every image I copied from my web could be dropped into any of the 24 boxes. Also I could move them around if I didn't like the position of any scan.

Lastly I reduced the size of the final image by 50% to get the size you see.

It really is simple