Can someone help me with what this says?
(see also: full picture)
I've tried Google image search, and even looking for different signatures, but no luck so far.
Usually this is the name/signature of the author. Note that usually it's not the original family name, because traditional Chinese people styled themselves some particular names or titles apart from the given name from the family.
In your case it looks like "古堂" and it's not a famous name to me at least...
=== Update ===
Regarding the signature, 堂 usually means "hall" or a large room used for specific purpose. I'm not 100% sure the first character is "古", which means ancient/old. Although some people did use the title of his unique place to present himself, yet "ancient hall" here as a place is kind of neutral to me to be used by a famous people...
The seals (which is very blurry in your photo) in Chinese painting or artworks do not have to be the name/title of the author. In some cases it can also be the name/title of a place, short phrases, etc. as long as it's identical to the author.