Just below where you type in your message, you will see an area saying
Attach File click the browse button, locate the file on your computer, and then click the open button. Then post your message. Do not preview the message, you may loose the image you are trying to post.
There is a size limit of files allowed to be uploaded, so if the image is to large you can resize it. I wrote an image resizer / thumbnail maker for resizing them, and it work in the windows environment only.
Here is the link.
Image resizer
Good luck, and give it a try.