First of all you have to have an image hosting site to save your images from your computer to. Most people use photobucket.com/ which is free to use.
First thing I do is download the photo from my iphone or camera to my computer. Then I have a photo program called Adobe Photo Deluxe that I crop and re-size all my pictures on before sending them to Photobucket.
I then go to Photobucket and upload the images that I want into my photo album that I created in Photobucket.
When you are ready to post a picture you simply open up the album in photobucket and copy the "image code" in the drop down box of the desired picture and then paste the image code on the thread where you want the picture to be displayed.