To insert of an image into a thread you must first have a total of fifteen posts. Seeing as you have over that needed amount, then you are ready to go. Now, you can either do a simple copy and paste of the image if found in places such as google images, etc or you can use the image code which Timberr covered above.
Just to note, some found images like those on Photobucket actually come with their own IMG Code so all you would have to do for that is copy and paste the code into your thread instead of manually inserting it yourself by typing. After you post, the image will appear.