Border or shadow for loading image previews

**Is your feature request related to a problem? Please describe.**
Some of my users find it rather confusing to have a blank area in the timeline while an image is loading. You can't really tell what it is or why it's there, especially for large images or slow networks, where such an occurrence is common and lasts a while.

**Describe the solution you'd like**
Since the image event has a width and height attribute, it should be possible to generate a shadow for where the image will be once it loads. This could either be just a shadow around the would-be outline (which could even stay post-load) or a background colour different to that of the timeline background, to highlight the presence of a loading message. A loader would probably be fine too.

**Describe alternatives you've considered**
All listed above.

**Additional context**
N/A