cache problem - not expiring

[Hajo Locke]

nginx 0.8.49

Hello List,

have a problem with cached images which dont want to expire from cache.
i use nginx as proxy with 2 ApacheBackends. On backend an image was updated 
by ftp but nginx still serves cached image for hours. i had to delete it 
manually...
in nginx.conf i use:

proxy_cache_min_uses       1;
proxy_cache_valid         200 5m;
proxy_cache_path          /usr/local/nginx/proxy_temp/ levels=1:2 
keys_zone=ramdisk:1000m inactive=10m max_size=2000M;

I need some help, in which direction i should look for problems?

Thanks,
Hajo