Problem

Currently, the backends treat icon images for the toolbar the same whether they're built-in, or provided by a user. This extends to replacing image.png image_large.png if it exists (#22135), and changing the black pixels in the icon on a dark theme (#22150). Both of these could be unwanted - in the first case, mpl can load a different file from the one requested, and in the second case, it modifies the image.

Proposed solution

Should we consider treating built-in and user-provided image files differently? Maybe by adding function arguments that control these two behaviours, having separate public and private functions for adding tools, or checking in the code whether the icon is being loaded from inside /mpl-data or a different location?