I would say that you should credit them, but I see no problems with posting it as long as you don't earn money on it. However, I would try to find other sources of reference photos, where you know you can use them (pixabay is a great site), or to ask the photographer/artist first.
Well, first of all, Pinterest isn't a source, it's an aggregation site. That means it's usually very hard to find the actual artist behind a piece if you found it through Pinterest.
Whenever you can, you should ask for permission to use other people's artwork or photography (I mean, photography is an art form, you can't just snag a photo and go 'mine now'). Many creators are friendly and give permission to pretty much anyone, or even have a blanket permission posted somewhere. (It's generally considered polite to link to their account/primary social media even if they don't explicitly require it.)
Finding the creator can be hard, but reverse image search and tineye.com are usually helpful.
If you want to avoid that whole rigmarole, you can search google images only for stuff that's marked as 'for reuse with modification' or #for commercial reuse with modification' if you're looking to make money off the finished piece. This will give you results that are either aged into public domain or were released to the public domain.
If it's from another artist a credit works most of the time. The original artist might take issue if they find it without a credit. But usually that at the most basic level is enough. You could contact the artist if you can find them, it is polite. Or not post the sketches at all and go with original works you've drawn. The risk either way is yours to accept or not.