To post a picture click the picture icon on the toolbar

 you will get a window like this:

Click the My Computer tab as I have done above, then browse for the picture you want to upload from your computer.

Click the name of the picture you would like to upload, then click Open.

Now click .

You can also upload the picture to a photo album, then right click it and select Copy and use Ctrl+v to paste it to your message.