There are two different ways to attach photos to an email on your iPhone.
If you are replying to someone's email, then the easiest way is to:
1) Press Home Button and go to "Photos". Tap on a photo you want to attach and hold.
2) Then tap on "Copy" box that will appear and it will will turn blue, which means the image is in your buffer.
3) Next go back into your "Mail" application and tap twice for the "Select | Select All | Paste" box to appear.
4) Choose "Paste" and all done!
Please note that when you attach images via Copy and Paste you will be attaching the full-resolution image to your email.
If you are sending a new email to someone and you want to attach photos, then the easiest way is to:
1) Open the "Photos" application and click on the Share icon in the top right corner of the screen (it looks like a box with an arrow leaping out of it).
2) Select the photos you want to attach by clicking on them to make a red check mark appear. You can select up to five images this way.
3) Then click on the Share button in the bottom left corner and choose "Email". This will open the "Mail" application with a new message with all the images you selected already attached.
4) If you select more than five images, the "Email" button becomes inactive, but you can still attach additional images by following Scenario 1 described above.