Use Own Avatars with the API

[FuckingToasters]

I use an MyBB Plugin which make use of the API to round standard images whenever an user don't have an avatar set.
In case a User set an Avatar, it shows as square image and isn't rounded the same so what i want to do is, to somehow submit user's avatar image to the API to round them correctly but i ain't sure if that's possible.

I tried to simply round them with CSS which however did not worked as expected (nothing changed)

Regards,
FuckingToasters