@odorex, I had the same problem and found out that it was caused by my initial meddling with thumbnail sizes on the catalog tab of the settings.
Here is the link to the setting page I was talking about:
(add your URL in the front of course)
/wp-admin/admin.php?page=woocommerce&tab=catalog
I had changed my thumbnail size for large icons to be 200×600 pixels because I thought it would be better for my products to be longer than wider.
Upon restoring the thumbnail back to 600×600 and then downloading the regenerate thumbnails plugin (to fix the oddities), my issue resolved. It had also fixed images in my Featured items, Top Sold widget areas too.
Here is a link to the plugin:
http://ww.wp.xz.cn/extend/plugins/regenerate-thumbnails/
I hope this resolves your issue.
