When you post a reply, one of the tools across the top of the editing box is a paperclip. Click on that an you will get a new window that allows you to upload files from your computer. Image files are limited to, I think, 100kbytes. Then go back to editing your message, hit the little down-arrow by the paperclip and select the attachment to go into your document. It goes in at the current insertion point (blinking cursor place).
Example picture:
Here's some more text after the picture.
No Spanish frescoes were harmed during the preparation of this answer.