Are there guidelines or rules as to just how much luggage pilots are allowed/suppose to have?

I assume pilots do not check their luggage, so where do they store it all?

I ask this after recently seeing a Delta pilot with more than the usual:

http://i895.photobucket.com/albums/a...030110_010.jpg

In practice, just how much do pilots need to bring? Since they are wearing their uniform...that leaves maybe one change of casual clothes, sleepwear, and maybe gym/workout clothes?