There are two main ways to include pictures in your messages.
1. Include the http link to a picture that is 'hosted' on a photo hosting site (their are lot os these e.g. Photobucket, PBase). When you are typing your message you will notice a button above the text box entitled 'IMG'. When you hit this it adds some code to your message. You copy and paste the http link to a hosted photo between the code and use preview to check it.
2. Now an easier method, if you scroll down from the message editor you will see options to upload an image straight to the forum and in to the message.
Your images need to be no wider / longer than 1000 pixels (1024 may be o.k.) and so you may need to re-size in photos in photo editing s/w. Personally I stick to 800 as a max size as it looks neater on my monitor.
Hope this helps