The add_stc is big in the report:

 Source: Auditory                                          |  7.5 MB
 Source: Visual                                            |  7.6 MB
 Source: Auditory/Left                                     |  7.5 MB
 Source: Auditory/Right                                    |  7.5 MB

Trying to fix this, I noticed that image_format='gif' isn't really supported properly. It is supported (I think?) when doing report.add_image, but for Report(..., image_format='gif') things are very broken. So I propose:

1. Add image_format='webp' format support for smaller file sizes now that matplotlib 3.6+ supports it: ENH: Add webp support to Report #11359
2. Remove image_support='gif' support for Report as a class
3. Add image_format=None kwarg to add_stc and add_evokeds, add_slider -- anything that supports a slider.
4. Make image_format='gif' work properly when making a slider by selecting frames from the GIF rather than changing to different images. (This should hopefully make the images smaller, at least in some cases.)
5. Add proper image_format='webp' support for animations by natively storing the frames in webp format -- this will hopefully be even smaller than GIF.

I already have (1) done because it's pretty easy. I'll open a PR for that, then later a separate one for 2-5 probably after doing some tests that (4) and (5) could actually save some space.

EDIT: Goal is to improve the ~60MB reports in mne-tools/mne-bids-pipeline#682