If the image is on a website, find the link to that image, and click on the "img" button and follow the directions. If the image is on your computer, use the "Attach a file" button.
If you find an image on a website you wish to display on GTP, right click on the image and click on Properties. Where it says Address (URL) is the link to the picture (http://...). Copy this and then place the following in your GTP post.
Code:
[im[b][/b]g]http://....[/img]
If you have a file on your computer you wish to put on GTP, you can either:
a) Upload the file to an image storing website such as Deviantart and use the IMG tags to get it up on GTP.