Hi Finn...let's see what's going on. Have you tried to quit both the photo and mail app? Sometimes, if even if you have done all the right steps-apps still act funny. To quit an app, hit the home button twice until you get the multi-task bar showing on the button. Find both the mail and photo app and long press on one of them until all icons begin to jiggle. Press the red negative button to both of these app. Than re-open the photo app/desired photo-to get to the box with the arrow coming out of it. Press that to give you the option to email or copy the photo to your email. See if that works for you.
Good luck.