I try not to post watermarked photos on other websites (unless I've edited them in some way) because a watermark basically means, "Don't post this image on other websites unless permission was granted by the original owner." But should I credit sources on pics that I found on search engines? I don't want to cause problems in the future when I post JF pics that aren't screencaps.
Most pics out there aren't actually owned by any website that's put them up. Ipstenu, I'm sure you can clarify this. But as for promo pics, magazine pics, etc. that are put out there, in my opinion unless they are owned and credited to a specific photographer or magazine, I wouldn't worry about crediting a web site that you found them on.