For fraks sake! Sorry, but i think i opened Pandora's box. I'm also really groggy so no do grammar good maybe.

1:does the file name look like xxxxxxxx_pX?
2:if yes, then it's from pixiv(dot)net
3:log in or create an account
4:copy file name from this thread
5:click on random picture on pixiv
6:in the url after it says illust_id= paste the file name you're looking for but delete the _px so it just takes you to the website instead of enlarging the picture
7:click on profile near the top left and you now have the artist

I hope this makes sense and saves someone time from asking for a lot of sources. Honestly, pixiv has a lot of guro under the R-18G tag so if you're into that...