Save the InDesign file as a png

[Max987letsgo]

I will export an InDesign file with scaled images  (collages) that all have 72 PPI. After scaling they got an effective value between 500 and 1000 PPI. What do I have to watch for when saving the png file, so what do I have to specify when it comes to resolution? Because I don't want to loose data. It shouldnt be a printable copy but a digital mood board. Please help 

[JonathanArias]

and why are you doing this?

[Derek Cross]

This seems rather muddled – firstly, how did you obtain your images for your collage?