What kind of trouble are you having? Click the "upload a file" button on the lower right side of the message and then select the image from your computer.
I usually use the copy and past method when at my PC. Right click the picture you want to use and select copy. Then in the text box, right click and select paste.
is this option disabled in private conversations? I am to post a pic in public forums but not in private....
For private conversations, you can upload to your media album and paste from there, or host on a third party site and post via the image link.