a) If you're using the brightscript plugin, sometimes it fails to include the new image in the package. Double-check the .zip file to make sure it's actually there.
b) I vaguely remember having some issue with list item highlights. I think this one worked though. You're using a list screen? I know I got something working with rolistscreen.
EDIT: For assets, I blatantly started with this here:
https://github.com/plexinc/roku-client-publicNot that I suggest copying them outright or anything like that, but sometimes 'what works' is a good starting point.