I find the easiest way would be to upload your pics onto something like photobucket (www.photobucket.com) then click on the button with the picture (like a mountain and sun) on the top when posting, then just put in the URL. Just make sure your pictures aren't too big
Hit 'Go Advanced' at the bottom of the posting box then hit 'Manage Attachments' near the bottom of the screen. From there browse to the image you wish to attach and hit upload. You should be right to go from there.