File does not exist: /home/username/public_html/domain.tld/media/catalog/product/cache/
It cleared the image cache.
media/catalog/product/F/U/
Your original images are still in place, so the clear image cache function obviously did not do as you first said, obliterates everything under /media/
Your major problem then is that your templates are not doing proper media helper calls to regenerate the image. Where exactly are you having your missing image problem?
A typical helper call asks for the url, if no URL exists, the image is added to the cache on the fly with the size chosen and then the URL is returned to the html block.
$this->helper('catalog/image')->init($_product, 'small_image')->resize(135)
Product View Page image
<img id="image" src="'.$this->helper('catalog/image')->init($_product, 'image').'" alt="'.$this->htmlEscape($this->getImageLabel()).'" title="'.$this->htmlEscape($this->getImageLabel()).'" />
Product Listing
<img src="<?php echo $this->helper('catalog/image')->init($_product, 'small_image')->resize(135); ?>" width="135" height="135" alt="<?php echo $this->stripTags($this->getImageLabel($_product, 'small_image'), null, true) ?>" />