We want to respect copyright as much as possible, in fact, we like to err on the side of caution and suggest peeps only use their own images, or images where there are clear usage rights spelled-out regarding attribution, etc.
I'm not a license expert, but from what I understand, there can be a lot of different copyright / license expectations and requirements that vary from source to source.
So, ya, I'd suggest erring on the side of caution and only use content from sources that clearly state permissions for how their content can be used.