Some random stuff off the top of my head:
PNG - black/white, make sure what you're saving either has a greyscale/b&w/palette (8 bpp) option and use it.
Don't use GIF, use 8bpp/paletted PNG. While you can save a few bytes sometimes, it's not worth it.
Screenshots and such should be PNG, nearly all of the time you should be using a palette/8bpp. If you use services like smush.it
, they will not try to convert for you, so make sure you save correctly the first time.
Diagrams - Where you have black/colour on white, it's often nice to make the white areas transparent, allowing readers with a background/page texture to not be harshly excluded. This however is seldom an option in vanilla editors, look for a 'white to alpha' or 'colour to alpha' plugin. I find the ones for Paint.Net quite good, Photoshop can be done easily enough with masks (but it's :effort
JPEG - If you're using photos, I strongly recommend using something like XnView
, you can often get quite a dramatic decrease in filesize while preserving details by playing with the subsampling factor in conjunction with the quality.
Don't posterize full-colour images to save space, e-ink has 16 levels of grey, but that doesn't mean your book wont be read elsewhere. The built in dithering is better anyway.
Remove your metadata, really - all of it. Colour profiles are actually used in some firmware, since the render packages often have defaults enabled, this is slow and generally useless. I've seen 20kb jpegs with 150kb+ of metadata/profiles.
If your book has many images in a single section - be very careful with using high res, iThings or not. A lot of newer readers (including eink) will use full RGB(A). Your 1k x 1k px image might only be a 30kb file, however it's 4.2mb in the readers memory. If you double the res, you quadruple the pixel count (2k x 2k = 16mb+ ). Not a problem for covers and such. But for sections full of pictures, you end up hitting performance hard on lower spec readers, and even on more beefy ones, slowing things down.
SVG - Tools like SVG Cleaner often save you some entities and such, which is a very good idea.