Add the image used to the manifest and include in the imageset/Duo directory, for example, PhoneApprove.png.

# Default manifest of all files that are part of the logon
# screens.
#ident "$Id: DFA.manifest 16042 2018-02-28 10:16:37Z phil $ "
#
# Format is "filename filetype".
# The first file is the initial login screen
# The second file is the logout screen.
#
lm_initial_dfa.html
lm_logout_dfa.html Text/html
lm_sso.js
espblank.gif
espbottom.gif
esptop.gif
favicon.ico
kmgerror.gif
kmgexleft.gif
kmgexlogo.gif
kmgexright.gif
kmgleft.gif
kmgright.gif
kmgstyle.css
PhoneApprove.png

When imagesets are created they exist within an imageset folder. Each specific imageset has its own folder within the imageset folder.