[FireAtlas] Take image size as default canvas size

### What could be improved

We could take the image size as the default size for new atlases that are loaded from an image.

### Why should this be improved

So that the user doesn't have to look up the size of the image.

### Risks

_No response_

### More information

_No response_

### Other

- [ ] Are you interested in working on a PR for this?

The issue does not name files or tests; start by locating FireAtlas and the code that creates atlases from images, then trace how the canvas size is currently chosen. Done means a newly loaded image atlas uses the image dimensions by default, avoiding a manual size lookup, with relevant tests updated or added if that area has coverage.

Written by the indexing model from the issue text.