Adobe has unified all its tools for creating stories with text, graphics, audio, and video into a tool they call Spark. The Spark app now includes the mobile apps formerly called Post, Voice, and Slate. These apps are now called Page, Post, and Video.
I just found out about this free app today. You can download it from the iTunes App Store. It is only for the iPad at this time. I created a little story and added images and a link to it in just a few minutes. You can search for public domain images from inside the app. You can read more about it here:
This site is free to use. If you use Adobe products you probably already have an Adobe account. If not, you can create one. You can browse resources by topic, software product, or grade level. I can almost guarantee you will find resources there you can use.