Your blocks should look like this(don't forget to add the permission for WRITE_EXTERNAL_STORAGE):
Create the web app as it is.
It could take a very long time to download all 600+ images, especially if the user is using data and not wifi.
Also, you should rename your first 99 files so that they sort properly e.g. :
page1.jpg > page001.jpg
page2.jpg > page002.jpg
page33.jpg > page033.jpg
page95.jpg > page095.jpg