I'm sorry for the long response delay. Somehow, I missed the alert for this message. My bad.
As for your question, well, I'm not very knowledgeable in this legal stuff, but as I understand ("as I understand" IN RED AND BOLD)
1. The zlib license applied to the code is really not applicable, since I'm not releasing the app's source code yet. But since I had to pick a license, I chose the one that looked the least restrictive.
2. The licenses applied to the images generated by the app are individually defined for each asset. They will be GPL 3.0 and/or CC-BY-SA 3.0.
In practice, what this means is that you must credit the original authors of the images (the credits file created with each image gives all the information about the authors).
Also, if you publish any modification of the images, you must also credit the original authors.
Hope that answered your question.