I have an iPhone 4 and it's really easy , at the bottom of the reply box there are three buttons , post reply, upload a file and more options
All you do is click upload a file , a screen appears with ,
Take a photo or video and chose existing
If you don't have a photo already in your phone click take a photo , this then sends you to your camera and you take the photo , this will then say use in the right corner and delete in the left you chose which one you want depending on your preference
If you have a photo on your phone you click , chose existing , this then sends you to your photo album and you chose the photo you require
Which ever photo you want is then underneath the reply box next to them are two buttons , thumbnail , and full screen , again you chose which one you want and they appear in the reply box
Then just click post reply
This is for iPhone though and may be different on other phones but I would imagine the set up is pretty similar