How many characters does your phone allow in a picture message?

With regular text I’m allowed 160 characters. However, I sent a picture message with text the other day and noticed that the phone allowed me 1000 characters (Yes, one-thousand. I had to do a double take.) What is up with that? And why can’t I get 1000 characters with regular text? I just checked again to make sure I wasn’t being stupid! Yes, 1000 characters.