"New" thumbnails as squares?

[caps20]

Hello!

I am currently moving my gallery to a new server and I want to change the thumbnails to be squares, because I really don't like the look of it the way it's now (Height is set to 75 pixels). It's not a problem resizing them when their shorter side is 75 pixels (..the height), I just cropped them with IrfanView to be 75 by 75, however, as you can imagine it is a problem when the longer side is 75 pixels because it doesn't come out as a square.
I've tried resizing them to make them bigger so I could crop them, but they come out with VERY poor quality (I ticked 'Use Resample function' already) ... and now I ran out of ideas about what to do.
I was wondering if anyone here knew a way to get all my current thumbnails to be 75x75 because I am really desperate already...

Thanks *a lot* for your help!   

[Stramm]

see sig

[caps20]

Thank you so much! First thing on my to-do list for tomorrow!