The National Wildlife Federation is obviously not the Digital Photography Federation, with such ridiculous and unnecessarily confusing rules. I don't know what... (edit: I've censored myself here - insert your favorite derogatory term here

) thought that it'd be better to give a dpi/size requirement than a regular pixel resolution requirement.
FWIW, those requirements mean that the photo can be no larger than 504x360 and no smaller than 216x360.
I'm sorry, I just can't take a photography contest seriously when they won't allow a photo larger than 504x360... that's
really small. Kind of like trying to decide the Oscars after watching the movies on an MP3 player.
Anyway, about your original question, I'd go with Irfanview, and make sure that it's resampling, not just resizing. (Resampling will give you much better quality.)