My apologies, to use the Image source type, use Image (singular) not Images:
Sources=Web+Image
I'm not 100% sure, but i think the Bing site uses InstantAnswer, RelatedSearch and Spell source types as well.
Bing does not have listed anywhere the exact parameters that the Bing.com site uses for searches. getting the API search to mimic the Bing.com search is a matter of configuring it to use the same parameters, since these are not documented, it's a guessing game.