I'm not sure about the correct way to deal with image aliases in custom modules / extension.
I've read this article http://ez.no/doc/ez_publish/technical_manual/4_0/reference/scripts/generic/ezcache_php and there is a image alias cache. Therefore I guess its correct that after clearing cache no aliases are available. But how to access the desired one?
The class eZImageAliasHandler has methods to check if a image alias exists and to get the full path to the desired image.