Editing thumbnail crops

Hi all! First of all, thank you for your marvelous work. Simple Catch is great and I'm considering an upgrade, if I can get everything I want to work on my blog. The thing I want to ask is if it's possible to edit how the post thumbnails are cropped. I'd want to be able to customize which portion of the full image is shown in the post excerpt. I've tried plugins like the Post Thumbnail Editor or Crop Thumbnails, but they don't work for me, they just don't save anything. It's getting really frustrating. I use the latest version of WP, Simple Catch and those plugins. The blog I'm talking about is http://olivermeres.hobbyart.sk/ Thanks for your help!