Pleased you found your solution.
You have to have some white in the image for it to register in the colormap.
Usually at the end and fades into the background so you think nothing is there. Tends to convert to not-pure-white #ffffff as well
Totally off-topic but using an indexed image was a way of concealing a message.
Edit the colors to hide the text, duplicate colours are not a problem.
[edit] excuse the error I have those the wrong way round.Then to reverse the procedure look for duplicate colours and assign a colour to them.