Once in Settings, Click on Custom Pages.
Select the Custom Page you want to edit.
Locate and click on the image gallery button.
Next Click on Choose File button and locate and select the image file you would like to upload.
Once you have browsed and selected the photo to upload click on the "Upload" button to process image upload.
After the image has uploaded to the image gallery. Click on the image thumbnail to select the image to upload into the custom page.
The last step will be to preview the image to be uploaded and scroll down to the bottom of the page and click "insert" this will send the image uploaded to the custom page being developed. The placement will be based on where your mouse cursor was last located.