First Step:
On the left-hand side menu: Select the Galleries option.
Click on Create New Gallery -> Enter the Name -> Add Gallery.
Click on the Edit button of the Gallery-> Add media Item -> Fill the details, for uploading the picture click on change ->Add Mediaitem
Upload all the pictures you wish to display by adding the media item for each picture.
Second Step: This needs to be done for the first time only until and unless you want to change the gallery it is referring.
Click on Manage Apps from the dashboard.
Click on the + symbol next to your app.
Create new Banner -> Fill the relevant details -> From Gallery list pick the gallery you want to display in Banner -> Add Banner.
Currently used attribute will turn green.