A widget like the Mx file dropper gives a small preview by default: https://marketplace.mendix.com/link/component/111497
It also gives you the option to directly upload and save the image if wanted.
With the default image uploader this is a bit harder and you could build a custom kind of preview but you indeed will need to commit the image first to be able to show it.
Hey Bart!
Here I have a video for you in which they show you how to do this with a preview photo.
Upload Photos Part 2: Implementing the Image Upload Widget - Mendix - Go Make It!
Although, in my opinion, a file dropper would work great in this case, you can test it here
Mendix Marketplace - File Dropper
Best regards,
Adam
/edit
I didn't notice that the question came up two years ago. However, I will leave the information anyway, maybe it will help someone :)