You can't attach a photo from within the Mail app. You have to go into the Photos app - the default one that came with the iPad.
Once there, tap the arrow that's in the upper right hand side. Then you can select up to five photos to include in an e-mail. Once you've selected the photos, press "email" - which is in the upper left side. Fill in the address details and anything you want to say and press "send."
It's a bit of a muddle, but since the iPad doesn't really allow you to browse the file system like a PC/Mac, it's what we got. Works well, though. I've never had anyone say they had trouble seeing the photos I send.
There are other apps that'll do the same thing. This one's just convenient, as it comes with the iPad.
Hope that helps.
Marilyn