oh i've recently looked deeper into finding these kinds of things for certain series
if you want to go through various blog sites on google to find the stuff, a good keyword to use is "コラ"
a website that trawls through a bunch of blogs and mirrors them in an english interface is here
http://ja.hentai-image.com/search/tag/photoshop/2chan is a great place to watch for people editing screencaps, but you need to be vigilant due to how quickly the threads disappear
there are plenty of sites that archive 2chan though
imgbako, futalog, futabalog,
2chin.net,
dev.ftbucket.infofor real time searchable 2chan, you can try ifutaba as a better interface for it too
if it's a current, popular series, you're bound to see multiple new edits per week popping up once you know what to search for
there's even some very dedicated fellow that is shopping cum onto pictures in a daily thread (at around noon jst, usually) for over 400 days in a row
