squished thumbnails

i thought i had this resolved but i am again seeing squished thumbnails where the aspect ratio of my thumbnail images is getting squashed so the image looks funny.

i took a lot of time to find out how to fix this about two years ago and for some reason i am not remembering how to do this or why it happens.

i am /not/ using images too small for the thumbnail and it is a problem that was solved but has cropped up again recently.

can anyone help please?

thanks.