Knights are less of a problem for me because you are rarely presented with just the card name, and rarely do you need to care about more than which one is on top. They also spend time in your deck and in your hand, so you have more opportunities to associate what they do with the name and image.
Boons, on the other hand, are trickier. The art is no help, since they are seen for such a brief moment and depict landscapes rather than people. Some of the names provide hints, as AJD pointed out, but I still have trouble with about half of them.
Here's the best I can do:
Mountain - gives a Silver, like from a dwarf mine or something
Flame - fire destroys your stuff, except your fortress of fortitude
Wind - blows some cards into your yard, then blows some cards out
Forest - you find the old Woodcutter in the forest, but he's old and depressed so he's not as good
Sea - the waves wash a soggy card to your feet
River - the river floods, but when the waters recede, a soggy card is left at your feet
Moon - it's dark but then the moon rises and you find that crappy card you were looking for
Sky - uhh.. it starts raining gold and you drop everything else you had to pick it up
Swamp - you tap one swamp and summon a flying, regenerating chump blocker
Sun - in the light of day you see how crappy your future is and decide to do something about it
Field - This one is just bazaar
Earth - umm, it's like a plein air workshop or something. I don't know. This one doesn't make much sense.