Originally Posted by JSWolf
Actually, you cannot. You cannot specify to use a specific image based on the screen size. You can't do "if it's an iPhone, use the smaller image and if it's an iPad, use the larger image" Standard ePub does not work like that. You have to build your ePub to fit most situations.
Sure you can. You can use media queries with screen dimension limits. I can't imagine why you would want to, but you can. Media queries are fully supported by both iBooks and Kindle, as well as nearly all other non-Adobe-based readers.
Caveat emptor, however. If memory serves, Adobe Digital Editions barfs on media queries, so you need to put them in a separate CSS file if you care about compatibility. ADE is violating the EPUB spec (where it says that readers must ignore unknown CSS gracefully), of course, and I've written to Adobe about it, but I'm not holding my breath for a fix. It's easy enough to work around by putting anything that makes ADE angry into a separate CSS file (and in the file that ADE doesn't ignore, specifying a set of default behavior that you can live with).
That said, I think it is well understood that ADE fully complying with published specifications is one of the signs of the apocalypse, so it is arguably good that they'll probably never get around to fixing the bug.