1) If you want to insert a picture already hosted somewhere on the internet, you right click on it and copy the URL (aka copy image address). Then back here on the forum in your post, click on the picture square of the editing options (next to the smileys). Insert the URL, click OK and done.
When you have the insert URL window open you will notice you can adjust the picture size. This is usefull if the image is too big to fit, it gets squished. So simply lower the values to something less than 800px or 600px. Keep the locket icon locked for maintaining aspect ratio. That makes the height change proportionally as you adjust the width.
If you've already inserted a pic and you want to edit the size, right click on the picture while editing your post, then select Image Properties.
Also in the Image properties there's a Tab on the top where you have options to add web links to pics and make them clickable to link to wherever you want to.
2) If you want to add your own picture that's not already hosted elsewhere on the internet. Then I sugges you register an account on imgur.com or tinypic.com or whatever image hosting site pleases you. Then simply follow the instuctions in 1). Copy pic URL bla bla bla...