Hi - your Cloud CDN charges will be in 3 broad categories → HTTPS cache lookup , cache egress , cache fill. In your case ; which component is causing the unexpected cost ?
if the images don’t have a lot of hits ( cold content ) – they will be removed from cache and hence when next request for same content lands on CDN → it will result in cache miss → goes to origin for fetch → and hence will attract a cache fill price component . Also , such transaction will also attract LB inbound data processing charge.
Hence architecturally what you are doing is possible .. but in my personal opinion , you may please need to consider the frequency of content access , regions of clients accessing the content etc.