What anime is this picture from?
If you have seen an 'anime style' picture, incorporated into a website's graphics, in an image gallery, as someone's avatar, or posted as a link on IRC, it is probably not from any anime at all. Most of these images are from pron games, or are just bits of artwork, cribbed from 4chan, Danbooru and similar
pools of degeneracy image boards.
For pictures montages in site graphics (as are so often used on fansubbers' pages) or other situations where the poster is contactable, asking them is going to be more sensible than bothering random people on forums and in IRC. If you must ask on a forum, try the AniDB forum.