The 3.5 app can be downloaded here:
Online Content Creator for 3.5
All content in one file can easily be downloaded here:
App forum for D20 Complete Reference and 5e Complete Reference
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At the moment there is no good way to do this.
This is on purpose because adding images would not only hugely increase the footprint (space required) for the app but also easily cause copyright issues.
The google image option is a workaround to provide at least some images in the application.
If you want to add an image to custom content in the online content editor you can do it by adding the image as base64 encoded in the html code editor, but it requires a bit of html knowledge and I don't recommend it with big images.