Juano, you can select the URL as the Image Source and then you can access the URL of the image property!
You need to stretch the image a bit out of the screen to apply it for the full screen. I guess this is a bug or this is how Adalo apply this because happening with web-view,map components too! 
Or I think you can do something like this instead of stretching the image out of the screen.
I’m not sure if this is possible with the Image component but for this there are some components you can use.
First one is the one tghat Pragmaflow built!
Pragmaflow Website
Instructions to download Pragmaflow Components
The second one is that Simon Siegert developer built and published on the Components Marketplace!
Thank you