Minimizing thumbnail sizes

>> Almost 2 MB of gif, can it be reduced?
>
> Btw I noticed some entries that already have a 2mb gif. If size is a concern, maybe all entries should be static-image only going forward? With time that page is only going to become increasingly burdensome for low-bandwidth connections.
>
> Allowing looped .mp4s is another option as those are a lot easier to optimize without significant quality loss.

_Originally posted by @erlend-sh in https://github.com/bevyengine/bevy-assets/issues/222#issuecomment-1189038859_

Related: #188

Start by reviewing the thumbnail and asset entries discussed in #188 and the original context from #222. Before implementation, resolve whether the project wants smaller GIFs, static images only, or looped MP4s; done would require applying the chosen policy consistently to the affected entries.

Written by the indexing model from the issue text.