In my wiki I solve all such problems by using the Wrap plugin. It allows to wrap anything into
- an inline HTML element <span> as so: <wrap> </wrap> or
- a block HTML element <div> as so: <WRAP> </WRAP>.
This way, pictures, buttons and likewise can be given a class and custom CSS. But, for insertion of a simple picture this should be easier and possible without any plugin...
So, what I find necessary is not another plugin solution, but, a tweak in the source code...