The content-fit option is only in GTK 4.8+.
A static file can't be set as a GdkPixbuf, which causes warnings when

  hovering over the avatar.
The custom icon menu didn't work as it only checked for an attached buddy.
Changing the custom icon didn't cause a refresh in the buddy icon; this is

  fixed now, but in a slightly annoying way.
Changing the custom icon in the buddy list doesn't change the icon in the

  conversation window; this is not yet fixed due to missing signals.

The latter two can probably be fixed better when buddies/contacts have better

signals.

Hovered over avatar in a conversation, which didn't cause any warnings.

Picked all options in Custom Icon menu from a conversation, and they all worked as expected (except for a few buddy list / conversation interactions noted above.)
Also, only tested IM conversations; probably broken in chat conversations, since there are no checks for it?