Navigate to File Manager in the Windows tab in the toolbar. File Manager Window should pop up if not already opened. You will find an add files button there. You have to add the image files in the project from there before you can use it in your code.
A few things about your code since you are using the designer:
1- You can set the activity title in the designer (no need to do it with code)
2- The ImageView background can be also setup in the designer (no need to do what you are doing)
3- Same for the rest of your views (Button, Label etc) you can do all that in the designer.
Navigate to File Manager in the Windows tab in the toolbar. File Manager Window should pop up if not already opened. You will find an add files button there. You have to add the image files in the project from there before you can use it in your code.
did you try to compile it in release mode? is the error still present there? you can alos force to use the assetfolder in debugmode with the following command:
#DebuggerForceStandardAssets: true
Next time please post the error message as text, not screenshot.
You don't need to use #DebuggerForceStandardAssets: true. Instead try to uninstall the existing app and then install it again.