Here's the way I post pictures (as well as most forum users on a wide variety of forums).
Save whatever picture you wish to post onto your computer.
Go to photobucket and create an account.
Upload the saved picture from your computer onto your photobucket account. Photobucket allows you to edit pictures in different ways and save them online. You also have the ability to resize/crop pictures as well as creating multiple albums for better organization.
Once you upload a picture, it will save into your album/account. Click on the picture. On the lower right, you will see a box with various links that would direct someone to that picture and/or album.
Copy the direct link (an example will be my signature taken from my photobucket -
http://i845.photobucket.com/albums/ab16/sirsoapmaister/RX7/Banner.jpg)
Now, when you reply to a post, above the text box you will see a variety of buttons/options. Click the yellow box that symbolizes a portrait of a mountain. This will bring up a dialog box, paste the link you copied from your picture on photobucket.
Another way to post a picture is by typing the code here shown in bold