Actually, its quite simple IF you're on a computer and have access/can locate your picture files. If you're using your cell/smart phone someone here might be able to help you.
Just locate the Upload a File button at the lower right corner of the text box. This will give you access to the various locations you have pictures stored. Since I don't keep porn pics on my computers, I use jump sticks to store my pictures and personal information. Find the picture file you wish to post, highlight it, and then hit the Open button at the lower right of the screen. It should upload it, and put a thumbnail pic of it just below your text box. Place your cursor where you want the pic to upload, and then hit Thumbnail or Full Image.
Hope this helps ...