I think for servers you should be able to manage permissions (ex. with ftbutilities). Because some players just post bad pictures on signs.

I agree, I've always had to ban vanilla signs so people don't create them lol

Definitely it's must have for using on servers. Please make sure it works ok with ForgeEssentials (if you decide to build this great feature ofc).

signpicture shows picture via sign follows format, so im not sure