One approach, that appears to be relatively markdown friendly, could be to place attributes in the currently unused pretty link position.
![[graphic.png|{width=50%}]]
Or just recognize that something like curly braces contain attributes
![[graphic.png{width=50%}]]
Alt text could be supported like this
![[graphic.png{width=50%}|alt-text]]
I’m linking this because it’s related and seems like pandoc has a reasonable approach, with the potential for additional utility
![[caption text]](graphic.png){width=50%}
or
![[graphic.png]]{width=50%}
This approach could easily allow alt-text
![[graphic.png|alt-text]]{width=50%}
As another option, I’ve seen something like this in the forums somewhere also.
![[<img width="80px">graphic.png]]