Pico8 carts are just a special flavor of png. I would try running it directly or if it won’t run them with the png extension just rename the file from .png -> .p8 without converting and see if that works

Relevant section of the user manual

There are three ways to share carts made in PICO-8:

1. Share the .p8 or .p8.png file directly with other PICO-8 users

Type FOLDER to open the current folder in your host operating system.

Although if you are having trouble you might have more luck getting started with the built in SPLORE command

Relevant section of the user manual

This might be easier to get started with since it will all get wired up automatically for you

Pretty sure you drag it to pico 8 to open it. The game is in the image. Then once it is loaded you can save how you'd like.