Is it possible to use a custom shape for proportional editing? I'm trying to add perspective to a picture however, I would like the proportional editing tool to affect only a certain part of the plane I imported. So, is there a way to pull only a specific zone (not a perfect circle), while using the the proportional editing?

Or is there an other way to easily add perspective?
Another method would be to select all vertices that should not be affected and hide them temporarilly H. 

It could be easier to select the vertices that should be influenced and invert the selection I.