Considering that the Japanese attitude in opinion polls have been that those men are good as dead and that they shouldn't be ransomed as they've gone in on their own volition, I think that mockery of ISIS is fair game in equal measure.
The artist is an anonymous vipper who posted the 406th comment on ”ISISを萌え美少女化してイスラムの奴らに送りつけようずｗｗｗｗｗｗ” thread of 2ch's "News for VIP" Board at January 25, 2015 00:56 [JST].
because the difference between a watermelon and a muskmelon in japanese is made with the words "Suika" and "Meron", the former being a Watermelon, the latter being the Muskmelon.
it says 日本の美味しいメロン in the image, which, given the previous explanation, refers to "Tasty japanese muskmelons"
