Add images to highlight

[waltersajtos]

Is your feature request related to a problem? Please describe.
For the new homepage design, we need images. We decided that adding the images to the highlight would be the safest bet so the images won't come directly from the average user.

Describe the solution you'd like
Add images to the add-highlight modal.

Additional context
This issue is blocked until the backend implementation is done. It might be possible to do this in parallel.
See #380 for some designs.