CSS not working

I’ve had this problem with my theme for sometime already.
When I create a css rule in my style.css file or custom.css file, it doesn’t work on my pages.

For example, I have created div class for my image on my post. I add the div around my image, but for some reason it does not take the rule from my css file. I did try “!important”, etc,. but nothing.
Also, I checked with Firebug, and it seems that for some reason it cannot detect the rule on the .css file, even my marking is correct.

I’ve been creating inline css styles for my pages and posts to get my styling to work, but I know this is not the proper way to do it.
Any ideas??