Document image inclusion in espass format

[eternaltyro]

I downloaded the example passes from espass.it and I noticed that the files named strip.png and footer.png automatically find their places in the pass display when they are included in the container. As far as I can see, this is undocumented in the espass spec. I can help with documentation for this.

[eternaltyro]

Seems to be hardcoded here: https://github.com/ligi/PassAndroid/blob/2eae832bc0ccd6f90b79a0dde9af6d0cfb2f0f4c/android/src/main/java/org/ligi/passandroid/ui/UnzipPassController.kt

[eternaltyro]

Please include dimensions of images also.

[eternaltyro]

I raised the issue in espass project page too: esPass/espass.github.io#8.

I'm leaving this open since it's partially related to PassAndroid app as well.