Sending iPhoto photos by E-mail

I took a still picture today with my camera. I wanted to send it to friend of mine through E-mail. Anyway, when I tried to send it, it ended up in iPhoto, and when I tried to then use the iPhoto E-mail portal, I was told, "The email server didn't recognize your username/password comboination." I didn't understand this, but I found another way to send it (picture to desktop>E-mail attachment), I can do this in the future, but I want to understand why I couldn't use the standard iPhoto route. What username/password and when? I wasn't asked for this today.