Nicole - your filter looks awfully full, I can't tell exactly since the pic was straight down, but my guess is you have about 3" in there - I would rather see 6 to 8" free space.

What may be happening too is that the dirt is washing through where the standpipe meets the valve - while you have it all appart, can you check for scratches around where they meet?
Also, it's hard to tell, but your gasket looks worn in one part - the straight piece above the part that looks like a "burn" in the pic - the "burn" is also raising suspicion pointing at blow by being a culprit, but like I said, I can't really tell for sure, but from experience with gaskets in water systems, the gasket may even be worn just a tad little bit (we're talking thousands of an inch here which is not noticeable to the naked eye) but that would be enough to cause problems.