We have encountered an issue with multisite and this plugin's cache.
Generally, our plugin directories are not writable, and whilst we can set 777 on a plugin/cache folder, we'd prefer not to.
For a multi-site installation, what are the implications of sharing this cache across multiple sites THAT MUST *NOT* BE CONNECTED OR SHARE DATA IN ANY WAY ?
What if 2 sites share the same image name?
Solution would perhaps be to put the cache in the blogs.dir/[blogid]/files directory some place? That way cache's would always be separate for each site in a WP multisite setup.