My images are stretched or too big, help!

I just started a wordpress.com blog a few days ago then decided to switch to .org. Everything is working out except how my pictures look: https://www.disorderlystitches.com/

Just scroll down the page and you’ll see they’re either too big or stretched. I want them to look like they did on my wordpress.com blog:
https://disorderlystitches.wordpress.com/
See how nice and uniform the thumbnails are? How do I do this??

Thanks in advance!