If you submit a book with small images, that *cannot* be converted to a book with large images, ...
On the contrary, if I submit a book with small images, it *can* be converted to a book with large images, because I have already done most of the hard work, which is finding and fixing almost all of the scannos. I think the idea that a book submitted to PG is "once and forever" and cannot be improved-upon by future volunteers, indeed is not even allowed to be improved-upon by future volunteers [!], perhaps using future better markup languages, or better scanners, or better sources found in some library or somewhere on the internet, well I think this "once and forever, no improvements possible" model is very sad and pathetic. What could be useful, and worth thinking about, is a way of caching large images for possible future use or consideration, perhaps as part of the submission process. Or even making the large images available on PG as backing files available to the public, somewhat analogous to the IA "All Files" directory. But, in the current submission process, the design of the "one size fits all" HTML/EPUB/MOBI triplet is by its very nature a requirement by the submitter to compromise. If, for example, HTML and EPUB were allowed to be submitted separately, then perhaps some more intelligent volunteer-implementer tradeoffs could be made, with the HTML targeting larger machines, and the EPUB targeting smaller machines. Please note in my current work process "the larger images" never actually exist, since I am working on images with an understanding of the goal I am trying to achieve, which is small compact images which still, hopefully, maintain somewhat the artistic integrity of the original work the way it was originally intended to be read. And in turn the current image storage formats we have available to us do not even provide good options for doing this, certainly not for woodcuts, somewhat analogous to the problem that HTML does not provide good options to do poetry -- or even reasonable dropcaps. [Or cover pages, or indexes, or ....]