POWL set size of images

I want to use a POWL to show a list of materials with a thumbnail. My problem is that the thumbnail is too big. How can I adjust the size of the image in the POWL result table? I cannot find any field for that in the field catalog.

Is it possible to access the ALV table model - that way I could set the height/width of the image column dynamically?!?

Thanks for your help.