Images posted on rgbstock.com are not in the public domain unless they are offered elsewhere as public domain images. They are not to be redistributed - displayed or offered for sale or download online, unless the gallery owner has given their permission.
I stumbled on some of mine being offered in a small size and marked as public domain on this site. I don't recognise any others, but I'll post the link in case.
I wrote to them, but they are most likely to ignore me. Please be careful even when downloading from a public domain site. Images may not be theirs to offer.
Here is another with my images and some rgb images. http://wesharepics.info/imagejgkl-january-birthday-background.asp
Surprisingly, the first site removed them.