Product Images are too big to fit

Guys,

I’m having quite tough time since last three days doing the right thing to place images of different dimensions in a canvas of 400x400px on single product page and 270×270 canvas on catalog page.
Below are the pages that are messed up. I just know that this can be achieved but how, that’s what driving me crazy. I hope someone can guide me how to deal with this situation.
https://trekkerstop.com/?product_cat=accessories&paged=1
https://trekkerstop.com/?product=shelf
https://trekkerstop.com/?product=halucination

Thanks in advance