Select Site >> Media Manager,

Then, click on the "stories" folder.

To upload an image you will need to browse your pc and then upload the image to the stories folder:
Once your in the stories folder, Select browse.
This will bring up a browse windows, select the image on your pc you would like to upload:
Click the image, then click on open:
The windows will close and the path to the file will appear on the txt box next to browse.
Click on "Upload"
Your image will be on the server ready to use in your page content.