Hi jonas,
In your navigation instead of using the open page activity, you can trigger a microflow that uses a create activity to create the "MyImage" object and the show page activity to open the page that you want.
Edit:
I recommend creating a page with a datagrid that has a data source of "MyImage" and have your navigation open that. Then in your datagrid, right click on the new or edit button and click generate page (for navigation layout choose popup). Now, you will have a page where you can view all images in the database, and when you click the new button, a pop up page will open where you can upload a new image.