Many of these resources are provided free for noncommercial and/or educational re-use.
Make sure you cite and give credit to the artist/author.
Many times this can be done simply with a caption.
Only if it is your own original work or it states "Public Domain" do you not need to do so.
Compress video files:
make sure they are mp4 and less than 30 MB
Draw your own pictures using Google Drawing
TMayo hint: download the pictures, open with paint and resize so it doesn't take too much space in your app.
Fonts:
TMayo's favorite code monospace font