Are you on mobile or on a computer?
If mobile, all you have to do is press upload on the right lower corner when you initiate a text. If you're on a computer, when you begin the text there should also be a linked text saying "Upload" in sky blue letters. When you click that, it's going to pop up your last files (documents, pictures, music, etc.) So just look for the file in wherever you saved it to upload it and wait until in loads and just write your text with the picture and you'll be good. 🏼