Is it possible to disable the download image sharing option?

You can  choose to disable image download in the gallery settings.  Click on the cog next to the gallery in the Layout Section to open the gallery settings and click on the Settings tab and check the disable downloads box.   Re publish the app and the app will no longer have a button to share images in the gallery menu. 

If this is the default way you wish to build your apps, save the layout and when you create new apps, the gallery will have that button checked by default. 

With the ability of people being to easily take screen captures, disabling the download button won't protect you from people being able to store their app photos on their devices. Most users do not turn off the download feature, but instead embrace image sharing and brand or watermark their images before adding them to the app.