Hello Olaf! Thank you so much for purchasing the full game! I sincerely hope you will show us some extra support by giving us a good rating and joining our Discord server! 😄
To answer your question, you can unlock the rest of the gallery by "rejecting" the True Ending! Consider the important choice you made to get the True Ending, and what choice you had to make in order to get to that point in the first place! I'm sure you will figure it out ;)