The first site steelhead refers to asks that you contact them first. Probably to be screened in some way. Some blogs and forums are that way. I belong to a few.
The second is a very easy site to browse. If you know the first letter of the artist name, select that letter and scroll through all the images cataloged thus far.
Neither is a definitive source, but both are good resources to have. There are many other resources for finding names to go with artist signatures, but you will have to do some homework to find them. If your item is an original or reproduced photograph, the list of professionals may well seem more daunting than that of other artists. If you can find other identifiers on front or rear of the media, or on the frame, that information could be helpful in narrowing any search.
Time for more coffee.