I hope there will always be small readers, as I find the size most useful to me. If I were to want to study closely some of the epubs I produce which contain maps, then the smaller size is not ideal.
Start with big images and mark them in your file system as read only so they won't accidentally overwritten. Then shrink them down to suit the need. Start with non-compressed formats (like tif) so that working with them won't create artifacts of being saved as something like a jpg in the end.
Zooming is reason to use larger images, but it will mean doubling the number of pictures. Even the PRS505 allows you to pan images so you can have larger ones.
I agree with Jelby on all but one point. If you save big images to begin with, you don't have to do anything but shrink them for a smaller format. And irfanview, a free program will batch resize them for you, if they can be the same size.
But the layout of a small book is never going to be the same as that of a larger one, for reasons I cited earlier. But a small book is not the best place for an image packed book in the first place.
One way to cut the Gordian Knot may be to save off the images for larger and smaller with almost identical names. Add a 1 at the end for the larger or smaller versions, then you could do a search and replace for .jpg and replace it with 1.jpg. That way you can use the same HTML and tweak for best appearance.
Last edited by mrmikel; 01-14-2013 at 11:38 AM.