Untill this sub-domain bug is fixed (images don't get cached) you can boost page load time with this trick.
All you have to do is to upload images to your main site. Than you call your images with long names (eg.
http://www.domain.co.uk/images/image.gif). This way images get cached and pages load a lot faster than before.
Still, this is just a workaround!!! I should mention that there are some other minor problems with this bug and hope that you'll "force" those at Ensim to release this fix ASAP.