Smushing thumbnail images

Hi, I have recently smushed all of my images which was a great start but I have an issue. My own Theme uses thumbnail sizes of 500x500px and it seems that the images on the homepage (500x500px) aren’t being smushed. How do I go about re-smushing these?

I have already resized thumbnails using the regenerate thumbnail plugin.

Any ideas?

Thanks

Tim