Image sequences (sometimes known as PNG sequences) are a powerful, but potentially dangerous addition to Motion Books.
They are powerful because you can include frame-level animation within a Motion Book story.
They are dangerous because it's easy to create a book that will be a gigantic download (gigabytes in size) and that will play with a very poor frame rate.
The best way to make use of image sequences in Motion Books is to either keep them small in dimension, or just a small number of frames.
The tempting thing to do would be to do a crazy animation sequence in an external animation program, export out the animation as a PNG sequence at 1920x1080 size, and then import the sequence into a Motion Book.
Keep in mind that PNG sequences are completely uncompressed. 30 seconds of 1920x1080 PNG sequence is
For a PNG with transparency, each pixel gets 24 bits, or three bytes. A 1920x1080 image has 2,073,600 pixels, so one single animation frame takes 6,220,800 bytes.
A typical animation frame rate would be 30 frames per second, so 30 seconds of animation contains 900 frames.
So 900 frames times 6,220,800 bytes is 5,598,720,000, or about 5.5 gigabytes.
A very useful way to think about image sequences in Motion Books is the "Cinemagraph". Basically, a cinemagraph is a photograph with a small portion that moves.
Here's an example that I created in the Motion Book Tool, exported as a video:
Now, you could create this by exporting a full-size animation of the whole page, but then, as we see above, it's a huge download and it won't run smoothly on smaller / lower powered mobile devices.
If, however, you only create a PNG sequence for the flickering candle, and leave the rest static, you still get some motion, but it's a tiny download, and it will run well, even on older mobile devices.
Here's another video with the non-moving part faded back a bit so you can see the effect better:
Here is a short clip of what it looks like in the authoring tool:
So, you can have lots of animations, and the overall impact will be that the book is more animated than it actually is, and it will download and run much faster.
Just keep the image sequences either very short, or very small.