Implement config for overlay application

[Larke12]

Given the sites focus on images it would be cool if there was support for images.overlay from the config.toml file to apply a watermark. This way a set image, size, and transparency could be selected, and the theme applies it to all images if it is set. Could also have it toggle to show only on the grid, or on both the grid and popup, or also for downloaded images.

[kc0bfv]

This would be interesting! There's a lot of possible configuration here that would be needed, but if someone implements this and keeps it simple, it could be a handy feature.