Most of my items are images, roughly in the shape of a postcard, and so the square thumbnails are really not doing a good job of portraying the rectangular images. I am working with the minimalist theme, and was wondering how I can alter the thumbnail default?

You can simply replace calls in your theme from item_square_thumbnail to item_thumbnail.

The "normal" thumbnail keeps the shape of the source image.

Can I piggyback on this question? I have the same problem but am using the rhythm theme on The featured items and recently added items shows a square version of the original image that often chops off heads etc. Is there anything I can do to alter this?