HttpHandlers / Modules :: Cache Images - Page Takes To Long Due To Images Not Caching?

I've been trying to get image caching working for the last 8 hours and I keep the same problem time and time again, I'm trying to cache Images at the moment and then when thats succesful, apply this to cache my js and css files. I have at the moment Sql Dependancy caching working on my dynamic pages but the page still takes to long due to images not caching.

What I have tried is going to IIS7 Management and adding HTTP Response Header for the images folder and setting various things for the cache control.i dont care how long it caches it on the client or on the proxy servers i just would like it to check when the file was modified and compare it with the one in the client or proxy cache and if they are different then fetch the new image which has been ftp up. i could solve this by changing the image name but the thing is the image name is generated when they are ftp'd to the site by an application and the name is a direct reference to the product so i cant change the name each time, as it would mean making a lot of otherpages accross the board.

so What i would like, is to set and expiry date of lets say 32 days or more on the images and then check to see if they have changed by date modified or some other way of checking(Etags maybe) I'm not sure, if they have changed on the server then redownload and recache them, I tried using post-check and pre-check together but that just permantly cached them, so when i change the image the only way it would display the new image is to just hit F5.

In Brief, how can i Cache images, on the client machine/ proxy cache and recache them when the modified date of the file has changed.