In several places, b2evolution lets you select an "Image Size" a.k.a. "Thumbnail Size" for the images you display.

In the name of the sizes you will find the following:

• type :
• fit will preserve the image dimensions ratio and reduce the image to make it fit into the specified dimensions
• crop will crop exactly to the required dimensions
• crop-top will crop by using preferably the top of picture (where the faces tend to be, when cropping vertical pictures)
• width (or max height in case of "fit")
• height (or max height in case of "fit")
• quality (for JPEG compression)
• percent of blur effect

If you need to change or add sizes, see the _advanced.php file.