If you can't edit an image, first make sure it's one that you've just inserted onto the page via the Image Tool. You can't edit images that are actually part of the paper background.
If it's an inserted image, you can edit it further by selecting the Image Tool again and tapping on the image. You can then resize or delete it. To crop or share it, tap on it again while it's selected and choose the relevant option.
Tip: You can also select an image by simply finger-tapping on it. See Select, move and edit content on the page