There's some talk on Twitter about what InDesign does to images when you export an ePub so I thought it would make sense to give a quick synopsis in more than 140 characters!
When you choose Export to ePub in InDesign, you get the choice of checking "Formatted" for images. What does that mean?
If you check "Formatted", InDesign looks at the images in your document and takes into account their current physical size on the page, and any cropping, shearing, or other effects that you have applied. It then exports the image at 72dpi.
This image originally measured 3264 x 2448 pixels. I both resized and cropped it until, as you can see, it measured 4" x 3" on the page in InDesign.
But the original size of the image doesn't matter, because if I check "Formatted", no matter its original size, InDesign will export it with a size of 288 x 216 pixels. (288 = 72 x 4", 216 = 72 x 3")
If you don't check Formatted, InDesign exports the images at the same size they had originally, in this case 3264 x 2448 pixels, and forgets about shearing or cropping. The one thing InDesign does do is change the resolution of the image to 72dpi, though, this is largely irrelevant.
Why is it irrelevant? I'm not going to get into a big discussion about resolution here (though perhaps one day I will), but I will tell you that InDesign can change the resolution to anything it wants, but the only factors that determine the output size of an image are its size in pixels and the resolution of the output device.
So, if the 288 x 216 pixel image is output on the iPad, which has a screen resolution of 132 dpi, it will measure 1.63" x 2.18". (288/132= 2.18", 216/132=1.63", and I admit, I checked it with a tape measure and it really does measure that.)
The physical size of that same image on a Mac, say in an eBook on Adobe Digital Editions would depend on the output resolution of the screen. It used to be that all Mac screens were 72dpi, but that's not the case anymore; for example, mine is around 98dpi. So, on my screen, the image measures 2.9" x 2.2".
Ah, you say, they look just the same! That's partly because I can only show them at a single resolution here—both of those images measure 288 x 216—and partly because everything else is in proportion and so they look very similar.
It probably makes more sense to decide on the size of an image by taking into account the full area possible. As I mentioned in Anatomy of an iBooks page, the usable area on a (vertical) iBooks page is about 560 pixels x 760 pixels. So, an image that measures 288 pixels wide will take up about half a page.
Please ask questions. It helps to know what's confusing. I'm feeling my way a bit as well.
And yes, that's our calf, Julieta. She's about a month old in this picture. We're getting about 3 quarts of milk every day and she takes the rest. She deserves a post of her own.
And to my non-American readers, I'm sorry about this "inches" business. I'd be curious to hear if you measure screen resolution in dpi, or not. I seem to remember using dpi even in Barcelona, where no-one cares the slightest bit about what an inch is, but it's been a while.