I've been colouring some of the images I sketched for the iBooks Authoring effort...I'm still not sure what sort of optimisation I need for the book, the screen of an iPad 3 is 2048x1536 pixels... at around 300dpi.. The advice on the apple support pages is to use JPGs, and keep the whole book below 2Gb...Aside from that it seems pretty open to interpretation. So I'm working at around 2048x1536 at 300dpi, RGB colour, and saving at 10 in jpg. This is giving an image size of about 400kb, so for 30 pages of straight book we are looking at 12 Mb... probably double that when I add widgets, which is a sort of ok size I reckon.
I'll have a look at what Sort of size other books come in at, it may be I can save with no compression, or as PNGs?
iBooks Author isn't really set up for building picture books... it wants to have Chapters and sections and table of contents. It seems the way folk have found to get round these issues is a bit of a bodge... but I'm happy to go with that.