You can use dropbox to host images that you'd like to use in the app you're building. If you don't have a Dropbox account, you can register one here:

Once you've created your dropbox account, we recommend creating a folder for your app to keep everything in one place. You can add more folders within the folder to organize these images as you please. 

Once you've created the folder, open it and upload all the images you'd like to use in the app. I've added three images to my "AppMachine" folder below as an example.

When you mouse over an image, a "Share" button will appear. Click on the "Share" button and a popup will appear. Click on the "Create a link" link. 

A link is created for the image and two new options will appear. Click on the "Copy Link" link. 

You will find that this produces a link like this one: 

This link will need to be tweaked slightly so that it can be used for hosting images for use in an AppMachine app. As you can tell, the link above ends with "dl=0" 

This needs to be changed to "dl=1". Your new link will be:

Paste this slightly adjusted link into your data (or paste it and adjust it in the data) and your app should load your images.