There are rules "Save to Cache" and "Don't Update" for these, so what's the point of having "Only from Cache"
"Only from Cache" works like the option "Cached images only" at Opera or the extension "Images like Opera" at FireFox. Only images that have been previously saved in your proxy's cache can be displayed when "Only from Cache" rules are active. This saves a good amount of page load time and your Internet traffic.
It's useful, for example, when you wanna read only messages on some forums. The skin images, background, interface buttons, smiles, etc. will be taken from the cache, but the new user's attached images will be blocked. This saves a page load time and your Internet traffic.