I'm moving your question to the InDesign EPUB forum.
You should reconsider if epub is the right choice of format. It is as it is, images need space.
You should target your images to 150 DPI resolution for ebooks. Now, we know how big they are on disk and their resolution. What are their pixel dimensions?
“I favor creating images at or around a longer dimension of 1,536 px at 150 ppi. By that I mean, in the case of a landscape image, it’s width should be no more than 1,536 px and its height should be whatever height results when scaling the image uniformly to a width of 1,536 px. If the image is taller than wide, it’s height should be 1,536 and the width whatever it comes out to be according to scale. For example, to create a full-page image that scales perfectly to an iPad Retina screen, the dimensions should be 1,152 × 1,536 px.
Why do I recommend 1,536 px at 150 ppi? For the following reasons.
- With a width the same as the iPad Retina width of 1,536 px, the image will go edge-to-edge in a margin-less view; when margins are present, it will scale down and increase its resolution.
- 1,536 px is two-thirds of the Retina height or longer dimension of 2,048 px, so if the device is rotated to show a two-up, or two-page view, the 1,536 px image is actually scaled down a bit, further increasing its resolution.
- Images that are not set the full width of the page in your layout will be rendered even sharper from the resolution increase inherent in the size reduction.
- It scales nicely to the most common screen widths and heights of 2,048, 1,280, 800, 768, 640, 540, 480, 320, and 240 pixels.
- If it scales up to fit the screen or is zoomed, the 150 ppi keeps the quality high.
- With such high quality original images, should you decide that you don’t need such high resolution images in the final ebook, the EPUB image export controls in InDesign have more original data to work with when downsampling and compressing, which allows them to produce better results.
Again, your opinion may differ from mine. The most important thing is to set a size and stick with it throughout the book. Otherwise images will appear in different sizes and the perceived quality (if not actual quality) will vary.”
Excerpt From: Pariah S. Burke. “Chapter 6: Working with Images and Multimedia in eBooks.” iBooks.