allow image replacement

[fgnievinski]

It'd be nice to allow replacing the image with another one of the same size (in pixels). Motivation: I've started my project with a grayscale image. Now, I'd like to replace the image with a binary version of the same image (with just the map lines on a transparent background). Currently, I have to download the control points (as a CSV file) then load it in a new project.

[timwaters]

Its a nice idea, and one its been looked at before.
Some institutions have multiple copies of an old map and they would like to update the base image. But in practice they seem to be all slightly different. The issue is that the replacement image has to have the same pixel dimensions and also have the same map/image represented on it.
This is because the control points use the x/y of the image space.

I do see the need for it for the reasons you give, where there some processing of the image and it needs to be updated. Lets keep this issue open (it probably wont be worked on for some time though!)