Okay. The picture has to be stored on the Internet.
There are several places you can store your photos.
Some are free and others you pay a fee for the use of their site.
Once ya get that handled, then you need to know the address where the photo is stored.
Copy the location of your picture.
Come back here and paste it into the body of your message.
If you look up at the area above your message there are several boxes, but various pictures in them. Bottom row, 1st box is the part ya need to put your picture in the message.
Right click the picture I have attached and you will see the words needed to post a picture. The 'http' is the link to my picture.
I haven't looked to verify this but I think there are some instructions on the top of the message listing on how to post pictures..