Click on the message board on the navigation. There are edited host messages and hidden signature files under the message board. Click Edit Host Message, and then insert the image.

If you mean leaving a message in another space, you can set it in your personal file and click.

Personal files, and then click on the spatial data. You can use at most one picture for a signature file, and the best picture size for a signature file is 540px* 160px. Insert an image in the box above, and then save the data.