I imagine the tricky bit is to print them to the exact right size?
Not exactly. It is a concern, and you have to tell all the software involved what the scale is and not to scale things from that. I scan the images at 200dpi. At higher resolutions I was seeing artifacts in the scanning. The scan produces a .pnm file, which apparently does not have scale metadata that gimp recognizes, although I could see that it had metadata with resolution_x and resolution_y as 200. gimp was having none of that, and it reported the .pnm file as 72dpi. I tried to find a way to convince gimp that the .pnm file was really 200dpi, but I never figured out how to do it. I then found a command line program (
imagemagick) that will perform manipulations on images. This command below converts the .pnm file to a .png file while marking it with the correct resolution and as a bonus, cropping the image from legal size to letter. On the .png gimp somehow finds the correct resolution.
convert overlay-200dpi.pnm -set units PixelsPerInch -density 200 -crop 1700x2200+0+0 overlay-cropped-letter-200dpi.png
"convert" is one of the imagemagick programs. Once the image is loaded in gimp at 200 dpi, gimp doesn't touch that value. I print directly out of gimp, and at that point the printer driver software wanted to scale the image down. The gimp image is 8.5 inches x 11 inches, and the printer driver knows my printer can't print to the full page margins. On the printer dialog for my printer there is one option that says scale which needs to be at 100% and another option that says, "ignore margins". The first time I printed I didn't check this checkbox, and of course the image scaled (very slightly so that it was not noticeable at first). With the checkbox checked everything is as desired. Of course it is important to lay the graphics out so that nothing goes into your printer's "no-print" margins.
If you don't print directly from gimp but instead send the image to a .pdf or some other intermediate software, then you have another piece of software that you have to watch like a hawk. But it isn't tricky; it's usually just a question of finding all the tick boxes and ticking them or unticking them as appropriate. I like printing directly from gimp mostly because I know that it will not change resolution in a surprising manner, once the correct resolution is set.
JR