Make it possible to "Magic Wand"-select a color that you need to crop away on a picture, so that you're left with only the image on the page, no white borders around it. (The same way it can be done in Photoshop or even Apple's Pages & Keynote)

e.g. Red dot on white square picture, becomes only the red dot that I can freely move around in GoodNotes, without hiding text behind the white border.