IS is probably converting the word dale to valley because that's what it is "hill and dale" is equivalent to "hill and valley", and few people actually use the word "dale" any longer. The keyword hogan also relates to a navaho structure (
Definition of hogan - Merriam-Webster Online Dictionary) and if you search that word alone you get quite a lot of images that fit the keyword (generically) - as a matter of fact, in the search I did, both keywords fit with roughly 90% of the images that were returned in the search.
In these two (IS & SXP) cases the keywords do not seem out of place.
The same search on Feature Pics brings up some of the same images that I got as a return for the search on the other two sites:
FeaturePics.com - stock photography : royalty free, rights-managed photos. Buy Photos, Sell Photos.
I can't really see this as being caused by some sort of 'worm' since the keywords fit the images.