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In Gimp, here is what I do and it usually gets results easily good enough for eink readers. (I always use the CBR/CBZ version for OCR, useless to convert their pdfs, they are terrible, and the CBR/CBZ images are nice big originals):
1. Crop the images if necessary (rectangle select, then right-click and Crop to Selection) to get shape the way I want it and get rid of big margins. (Using a CBR/CBZ you are opening the whole page as an image).
2. Right-click and Scale Image. IA images are usually huge. For a full page image I use 1200px as the max height. Resolution at 100px/in or better. If a map, diagram, or something I may want to zoom in on, 300px/in or more. It gets maps you can actually read on eink.
3a. Right-click and use Colours-->Curves. Drag the right side of the curve up to lighten, drag the left side down to darken. If the image isn't too brown or yellow, this is often enough. Experiment!
OR
3b. Right-click and Colors-->Desaturate-->Colours to Grey. Then use Colours-->Curves or Colours-->Brightness-Contrast to get a sharp contrast that will show up well on enik. I use this if the image is very brown.
If the image wraps around text, I simply use a paint brush to eliminate it. Ctrl-Click to pick up the background colour, then over-paint.
If the image is very speckled, a paint brush can help, or go the layer route as Jellby suggests (I've never tried that, sounds good, if complicated).
If you have an image, like a title headpiece, that spans two pages, you can easily put them together in Gimp, too. Just open each image, and crop just the left and right margins so the images are at the edge. Export the two images as new files. Then do File-New and make an image as wide as the two cropped images together. Then do File-->Open as Layers and select your two cropped, exported images. They will open on top of each other, so hit "M" (for "move"), and drag them so the edges match up. Then you can re-crop, fiddle the colours, and so on, and export the result.
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