It probably sounds a bit kludgy, but I have in the past used OneNote to build a larger image out of smaller component parts, using SC (ScreenshotCaptor) for the trimming of the parts, where necessary. The user can move the images around either in the same container, or in separate and discrete containers, and abut or overlap them, as necessary, so you can't see where the edges meet. Then capture and copy the composite as a whole in one Copy command and save it as an image in (say) irfanview, or something. A bit fiddly though, and. from memory, it's easier to abut images placed one above the other, than one to either left or right of the other - the latter probably need to be overlaid in separate containers.
I'd suggest a suck-it-and-see approach as a trial, using OneNote. I think you might be able to effectively have an infinitely large - or very large - page, if you want.
There's an example here where the work to be done was rather tricky and a high degree of precision was required, so as to result in what had to look like apparently plain print, which could then be passed through an OCR scan with minimal errors in the output: Align image sections - 2 half-page image clips of a page of a document;then OCR.
There are quite a lot of other instances in that discussion thread, where the fiddling about with images would not be apparent to the reader. To a great extent, OneNote helps to minimise the fiddling about and can be a real timesaver. SC can be an invaluable aid in that process.