I would look at the Kentico’s Dancing Goat image upload widget they have provided as a first pass to see if it delivers what you need.
You can get this via the Kentico site - trial Kentico 12
They have an inline widget that does this.
If this is not what you require then there are other custom options available online.
A simple version is shown here - upload image and preview
Then it’s important to write the functionality to save this uploaded image.