A note for those curious, the reason it's Spider-Man is apparently 2chan, and numerous other places now, decided that Astaroth has a girl hard-on for Spider-Man. And T-shirts.
If doujinshi.mugimugi is anything to go by, the background is from a doujin called "Wearing A Rose-Coloured Shirt" (バラ色のシャツを着て). I haven't been able to find it yet, though.