I've looked around for answers, but none of them seem to apply to my case.
The product's images were changed via Magmi with a csv file with the following structure:
"sku","image","small_image","thumbnail"
They were changed from png to jpg files.
I've refreshed all 7 cache types, I've flushed catalog images cache, I've flushed Magento cache. I haven't flushed cache storage because the warning message scared me...
If I open the product in the admin area I see the images are properly set. If I save the product, the images appear correctly in the catalog view. This leads me to believe it's a problem with indexes or temporary files. I was thinking it could be the Category Products index, but Magmi updates that while importing products.
I've cleared the browser's cache... I'm out of ideas.