Looks like this plugin dont like this syntax for image
<img src="/img/button.gif" />
You will get this warning, because the plugin dont add the
current blogs uri! The plugin then creates an image path like this.
:///img/button.gif
Here is the complete warning
Warning: file_get_contents(:///img/button.gif) [function.file-get-contents]: failed to open stream: No such file or directory in /mnt/....../hot-linked-image-cacher/hotlinked-image-cacher.php on line 219
http://wordpress.org/extend/plugins/hot-linked-image-cacher/