giving credit

File: do not undersand.gif (36KB, 720x720px) Image search: [Google]

36KB, 720x720px

so I've never really been part of any arts communities , and I have heard you can get into a lot of hot water because there are no set rules.
I'd just like to get everyone's two cents on this

I make spray-paint stencils, and I'm not really sure to what extent I should credit my sources
>always and for everything anon
but it gets grey in my head at least pretty quickly

if I make a stencil of a photo, is that stencil a copy of the photo; or a derivative work?
does the medium of the original stock make any difference? (if it was another stencil surely I would just be stealing it, but what if it was a print, or a photo?)
if I combine a number of images together, does it become my art or have I simply multiples the number of people I need to give credit to?
what if the stock image was not "art", but for informative value (like a product photo)
what if the original artist was dead or lives in iceland?

if I credit someone can I use someones work if they don't want (or havn't authorised me to)?

how the hell do I credit someone in a piece of way two stories up, especially since I don't have an official catalog or website?

like I don't want people to think I'm stealing their work, or trying to claim talent that isn't mine
but at the same time my work is generally illegal, so It's a huge risk asking anyone for permission; and I'm not sure how much effort I'm expected to go to crediting people who may never see my work