It's a security measure. I think you have to post something like 2 or 3, or maybe it's 5 messages. I'm sorry I don't remember the number. But after that, you'll be able to create a signature.
I only know the rules by personal experience. They're not stated anywhere, that I know of. Images are restricted to 60 pixels. It does tell you that but if I recall, only after you try to insert one that's too big. Text is limited by the number of characters. But it doesn't tell you when you're about to go over. It tells you when you post, that you have too many, so you have to just delete until it accepts it. But you can use my signature as a guide. From what I've been able to determine, mine is within 5 or 10 characters of the max. To make that tiny image fit, I had to use one particular host, which creates short URLs!
Note that the links in my sig ate up the characters, because of the code needed to make the links. So if it's straight text, you can use probably 20 or 30% more characters than my signature. But that 60 pixel image restriction.....well, it does keep the forum free of some of the dark and disturbing images that I've seen are popular in some graphics forums. And I don't know what exactly is possible with this phpBB software. But I wouldn't mind some looser restrictions. Although it might be more related to the size of the hosted forum, that detemines how many images can be posted. I really don't know why the signatures are so restricted.
Anyway, you'll probably have a signature in no time