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In any case, if the cache directory is writable but you see blank thumbnails, the problem is most probably related to file owner permissions. In other words, the cache folder may indicate "writable" in Joomla, but no Joomla plugin or other extension can actually write to it.

In this case, if you don't control the server yourself (e.g. your site is on some shared hosting plan), ask your hosting company to repair "owner permissions" for your entire site. This will solve the issue. But if the issue persists in the future (when you install other extensions), you need to ask them to do this again. If this indeed happens, then you should switch hosting companies because this clearly indicates that hosting company has no idea how to properly setup a server for any PHP based application (like Joomla).

As a sidenote, if your hosting company is Rochen, check that they are not blocking writing to the cache folder (i know they enforced this on all Joomla sites a year ago or so).