Put an image on a material

Hello,

I have a question that stay without answer. Is it possible to put a png on a material and to place it where you want on the mesh ?

For exemple, I put a material on a wall, and even a second material and now I want to put a poster on it.
Is it possible to just place it where I want ? I don't want to create another material, and I'm stuck with a superposition of 2 materials. Sorry for the poor English, I hope it was enough clear !

Example : I just want one BOOM, and I want to be able to put it in the left-down angle of the mesh.
How can I achieve that ? My models are from sketchup, there is no UV maps to paint them.

The only solution I find right now is to put a quad and put the BOOM material on it, with transparency. The result is ok, but it only work on plane surface ...

 

How about using a decal system:
https://www.assetstore.unity3d.com/en/#!/search/page=1/sortby=relevance/query=decal

 

Thanks