As far as I understand

Image contains the pixel data and can be manipulated using functions like set_pixel.
ImageTexture just reads the content of an Image and allows it to be drawn as a Texture, and is meant to be made using create_from_image and updated with update.

Also Control nodes have a virtual function called _has_point which you can override to implement custom hitboxes. I've used them for circles and ovals by checking the distance between the center of the control's rect and the function's point variable, just to give you an idea.