Upload your pictures onto a file sharing website like photobucket or flickr or whatever. From there you can copy a url link to the picture. Then when you want to post the pic click the IMG button just above where you type the message and paste the url link in the popup window. And then you're good to go.
Attaching a file is good if your pic on your hard drive only, and is of the correct file size, <140k and type. The file I have attached as an example is a jpg and only 25.7k so it will work easily.