You need to open an account with imgur, or another pic hosting site. Download your pics there and copy the BBCode and paste it where you want to in your posts. Like this.
If you want to try different sizes then you can use the Preview button next to the Save button, it shows the preview above the text box.
Width=500
Width=400
What I do then, is after I've pressed save I goto the post that I've just done to see if the Pic is too large. If it has scroll bars I just modify the post and change the width= number. And then Save again. But that is just my preference.
Hope this helps.
P.S. Sorry about the picture I've used. I just find it cute. Hehehehe :
P.P.S. Just remembered as well I tend to highlight what I've just typed and copy it. Sometimes when you press save it doesn't work. It saves you typing it all out again. (Just paste it back in). It hasn't happened to me for sometime, but it's worth doing if you can remember.
P.P.P.S If you want me to remove this from your thread just say.