Auto resizing thumbails using custom CSS not working?

Hey there Forumers.
I have a Price Drops section on my website http://www.htarcade.com which includes a thumbail picture of the game cover next to the game that has its price dropped.

For the last week or so I’ve had to manually resize the image in Photoshop to 40px X 40px . However I need my staff to be able to update this section easily while they’re at work and a lot of them don’t know Photoshop to auto resize the image. Not to mention that uploading a game cover and having the CSS auto resize it would be much more efficient.

I’ve tried using a custom “.thumbail” class that is set to 40px X 40px but the class WordPress automatically generated,”attachment-post-thumbnail wp-post-image” seems to override it. I thought this would be an easy solution and simply create and adjust the “attachment-post-thumbnail wp-post-image” class myself but it evidently did nothing.

I’ve looked at the options when setting a “Featured Image” in the WordPress dashboard but their “Thumbnail” size is 200px X 200px.. which is still far too large for what I’m after.

Any Tips and advice is greatly appreciated! Thanks a lot.