Image dimensions are globally controlled in K2's Settings ("Images" tab). After that, you can override them per category, in the "Display Settings" tab of each K2 category.

The size you're viewing is called "Generic" and apparently you set it to 75px at some point in your K2 Settings.

If you have lots of images with wrong dimensions, you can rebuild them using this script: github.com/getk2/rebuildK2imageCache

Or you can update your user.php override and fetch another size, e.g. "imageL" (for "large").

With K2, you'll always have lots of options to do things ;)

Yes, it looks like CSS resizing is applied.