Images are not refreshed when they are changed or deleted

My app’s URL:
I have refreshed/deleted some images but the App still show them.
You can reproduce the problem in this way in may app
Open APP -> Select “Orari” on bottom right corner
Than select “Orario Docenti” and choose one of them.
You could see an image, but that image no longer exist.
Infact if you tap on the image (for open the url in browser) you get the error
this was the URL of image that is still showed but no longer exist.
I don’t know what is the problem, but seems to be a cache problem.
Could some one help me ?
In this topic someone had the same “problem”
How can I cause my images to update whenever they change?
But i cannot reach to understand how it has been resolved.