Are you on a desktop Travelsafar?
If so, click on 'choose file'.
That should open pictures on your computer.
Open the file where the picutre is that you want.
Double-click on the picture.
You should see the file name for the image next to 'choose file'.
When you post your message, the image should appear. It sometimes takes a little while.
It's easier on a smartphone, when you click on choose file it should take you straight to your picture gallery, just select your image and then 'done'
Hope that helps.