Well, I just clicked every link I could find on your site, and all the images (logo, background horse in header, and embedded images) came out fine.
It's got to be a cache issue - though sometimes figuring out what is cached where can be frustrating - your ISP could also be caching. Additionally, FireFox doesn't always completely clear the cache, even when told to do so, and IE has it's own way of caching certain things.
To be completely sure it isn't a cache issue, you should clear the cache, and then completely close your browser, then reopen and browse again.
I have not looked at your site in IE - it's always possible tha IE has mangled your CSS - but that should not apply if it worked before the .htaccess change. Trust me on this, it's a cache thing.
If all else fails, clear caches, close browsers, disconnect form net...then connect and browse again - I'm betting it will "fix itself" if you do that.
Of course, you could have something very weird happening - write back after you've tried those things lwt us know, ok?