If you want to post a picture, in the Critics Lounge for example, you will need to have a web space account to upload it to. The forums themselves do not provide a facility for you to upload pictures. One useful host is imgur.com, where you can upload JPEG or PNG files for free and select the code "Direct link to layouts" to receive the link to your image.
Once you have the image uploaded to the internet and know the complete URL of how to access it, you can use the following forum code to include the image in your message:
where the italic text would be replaced by the URL to your image.
NOTE: If your image is large (i.e. wider than 800 pixels), please use the tag in this way to resize it: [img width=800 height=600]https://www.website.com/path/image.png[/img]. Having large images in posts makes reading the thread very difficult for people running lower screen resolutions, and resizing the image in this way will make it fit comfortably for the majority of people.
NOTE: You should always save your image as JPG, GIF or PNG. BMP files are uncompressed, and are therefore very slow to download, especially for people with slower connections.