Image Gallery Plugin support for thumbnails

**Motivation**
Image files can be big, very big. In the case where large images are displayed in the Image Gallery plugin they can take a long time to display, especially when there are many of them.

See example here where it is taking ~15 seconds (with a good internet connection):

https://bbp.epfl.ch/nexus/web/public/thalamus/resources/https%3A%2F%2Fbbp.epfl.ch%2Fneurosciencegraph%2Fdata%2Fdensities%2F3ae5fcb0-c451-4352-8321-0eb49b555989

It may not be desirable to compress the original image. Image thumbnails should be supported such that in the resource an accompanying, reasonably sized compressed image, can be included along with the full size image and be presented to the user for quickly previewing the image.

**Acceptance criteria**
- [ ] Large images should have thumbnail

Start by locating the Image Gallery plugin and reviewing the example resource URL in the issue. Trace how large images are presented and how a resource could include a reasonably sized compressed thumbnail alongside the full-size image; done when large images show a thumbnail for quick preview.

Written by the indexing model from the issue text.