If you just want to add a picture to your thread, go to the site that you have uploaded your picture to.
Pick a picture
Right mouse click on it
Go to properties
Highlight address url. Right click again and select copy. Place pointer over the highlighted part and then left click on copy. Make sure to copy from the http at the beginning, all the way to the end
Copy address
Come back to your post
Type
Paste the address with no space between the [img] and the http in the address
Type immediately after the full url address
Click submit