Adding support for media files is rather complicated, as:

• we will want to restrict all uses to Wikimedia sites, i.e. upload.wikimedia.org;
• to get the raw image, you need to MD5 the filename and then chop it up (e.g. File:Wikipedia-logo-v2-en.svg -> Wikipedia-logo-v2-en.svg -> b3c6a567391e03d0b4eb780aeae24eb5 -> b/b3/Wikipedia-logo-v2-en.svg;
• for media uploaded to Commons, that's then reliably https://upload.wikimedia.org/wikipedia/commons/b/b3/Wikipedia-logo-v2-en.svg — given different wikis' copyright policies around fair use, that's probably all we'll support;
• for non-largest thumbnails (which we can't know in isolation), that's controlled inside MW in a black box that turns requests into e.g. https://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Wikipedia-logo-v2-en.svg/120px-Wikipedia-logo-v2-en.svg.png;
• for SVGs we proxy ~all user image requests to a down-rendered PNG instead;
• for timed media (video files, audio files, etc.) we also would need to ship the player and extra information; and
• we'd want to provide a convenient mechanism for Function authors to trigger all this without having to think too much about it.

It has been simplewiki’s policy for twenty years now, so I wouldn’t expect it to be an issue.

https://simple.wikipedia.org/wiki/Wikipedia:Image_use_policy

• I have no issues with the restriction to Commons as our only source.
• As a first pass it would even be okay if it excluded timed media, or even SVGs.
• The rest sound reasonable.