How do I use my png file as a Shortcut icon?

See 3. on this page from the Shortcuts User Guide:

Add a shortcut to the Home Screen on iPhone or iPad

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Hello ShinehahGnolaum,

Apple offers some insight on this subject. Check out the information offered here on this page: About transform actions in Shortcuts on iPhone or iPad.

Transform actions edit content (such as cropping an image) or convert content to a different format (such as changing a PNG to a JPEG). Using the Content Graph engine, Shortcuts often automatically converts content that’s passed between actions, but you can use transform actions for more control over the conversions.

As you build your shortcut, you can search for transform actions using keywords like make, filter, convert, edit, get, and adjust. For example, you might want to use Filter Images to separate out portrait photos, or use Make Rich Text from HTML on a link from Safari. A search using the keyword convert displays actions that perform conversion tasks.

Other helpful transform actions include Combine Text, Combine Images, Translate Text, Detect Language, Filter Event Attendees, and Filter Files.

We hope this information helps you out. If you need further support, you may want to reach out to Apple for support directly. You can reach out to them by clicking here: Contact Apple Support or over the phone by calling the number that is most local to your region: Contact Apple for support and service.

Best wishes!