Add photo upload feature for sub-tiles as well

[Gergely Rakoczi]

In order to build visually harmonious courses

as trainer

I would like to be able to upload images for the sub-tiles as well.

Current situation:

Currently trainers can upload individual images to tiles only in order to customize their course appearance. For sub-tiles this option is not available. However, this would be truly useful to create harmonious course designs across courses or even the Moodle installation.