search-portfolio-thumb => actual thumbnail, not whole image?

Hello,

I’ve noticed that the “thumbnail” which appears to the left of a search result/category entry is actually the whole featured image and not a cropped/resized thumbnail. Given the fact that most of my pictures are actually over 1000px on the long side, I would like to know if there is a way to have a proper cropped thumbnail appear in their stead.

Thank you!