Since this is posted in the Slim Plus template support forum I’m going to assume this question relates to this template, yes? If it doesn’t please let me know.
Slim Plus requires a specific image size in order to maintain its layout design. The size is 100x120px. To achieve this Connections will scale it so one of the image dimensions, the smallest scaled dimension, to fit and then it crops the largest scaled dimension of the image so the new scaled and cropped dimension are 100x120px. This is all based on the image’s center.
I’m certain this is what you are seeing, but if it is not, you’ll have to share links with me showing examples of where it is not and probably the original source images so I can see if I can replicate the issue.
Also, this FAQ might help.
Let me know if this helps!