If you wish to upload many images items and/or replace them in the content.  

- Select the Upload Many option in the Item section of the App Designer.

- Once you have selected Upload Many, choose the Import Type you wish to accomplish, Replace All Items or Add Additional Items.

- Drag and Drop the images you wish to upload on to the popup-box that or Click on the box to select the file in-which the images are held.

- Hit Continue once they have all be uploaded

DONE 

Additional information

- If you wish to change the order in which the image(s) uploaded on to the content item(s).

- Go to the Edit option for item(s). 

- Click on the drop down arrow for the Image, select the image you wish to display/change. 

- Hit Save at the bottom of the Item content box.

Done