This is my first time doing this. So, does anyone know how to put a picture in a discussion forum?

Upload it to a image host and put it in the [img][/img] tag.

Upload your desired image you want to use to an image host the Scratch Team allows (for example, Cubeupload). However, you must make an account.
If you don't want to make an account for those image hosts, then make a new project, put the image in the project (using it as the thumbnail), then go to My Stuff and copy the image address of the thumbnail and put the address in the img tag

use an image host and an img tag

The two most common image hosts are Cubeupload and Scratch itself. Scratch is the most reliable, but it's kind of difficult to use it as an image host properly. Cubeupload sometimes goes down, but it's easier to upload to. Don't worry about the account system, the accounts are only to keep track of what images you have already uploaded just in case you want to use them again.