Go to WooCommerce → Settings → Catalog.
On bottom of this page, please make sure Catalog Images and Product Thumbnails have image sizes setup as stated below:

Catalog Images: 280px width and 180px height
Product Thumbnails: 80px width and 80px width

Note: If they are not as stated above, please edit them and set the above values.

Read more about product images and gallery settings here: http://docs.woothemes.com/document/adding-product-images-and-galleries/

Tip: To perform bulk automatic image resizing for all images across your site, install a free plugin called 'Regenerate Thumbnails'. Please note that automatic image resizing should be done only after you have done the above step (i.e. applying image sizes)