You cannot alter the color of an image via Sigil. You need photo-manipulation software for that, completely separate from Sigil. Try Gimp, which is free OS.
Second, with regard to your question as to how to add an image, I'd recommend that 1) you read the Sigil tutorial, which explains how to do it and 2) that you learn some html, which you'll need to do anything above and beyond what everyone has already told you how to do: try this:
W3 Schools html tutorials.
Hitch