facebook is having trouble caching icons served by elgg 2.3

i have code on my site that adds metatags to relevant pages so that when the page is shared on facebook etc. - the correct image is scraped and used in the shared object when it is displayed on facebook. i notice that since elgg's icon handling has changed, facebook usually fails to get the right icon and i think this is possibly due to HMAC mismatch. is it possible to serve an icon in elgg that has no session data / codes / ids attached to it in any way? so that the url will always be reliable to be used by many users and thus is also suitable for distribution to social networks?