I found that there’s some kind of problem with the form in which we insert the image. For some reason the file inserted in that input field is not recognized. It’s like submitting the form without having selected a new image for uploading. That’s why the old image is not replaced with the new one.
The function handling the image replacement is called updateBannerFromLocal() and you can find it in wp-bannerize/Classes/wpBannerizeAdmin.php file. In case you want to test it yourself.