I think the
Dragon Magazine parody of
American Gothic (though I haven't seen it, only read about it here) is okay because it's parodying the subject specifically (i.e.:
American Gothic). However, the manner in which the Rush Limbaugh Newsletter is using that photo isn't parodying D&D, that monster, or anything associated with RPG's. Instead, it's using the picture to show Rush Limbaugh as a lone warrior bravely facing off against a representation of Barack Obama's administration and/or election campaign...in other words parodying Barack Obama as a gigantic monster, and has nothing to do with D&D, the specific monster, or RPG's in general.
That's not a legitimate use of parody.