When working in Drupal 8.7.1 or Drupal Web Application, sometimes you might encounter an error where you or any other users are not able to upload images to the site.
Below are possibilities that might be happening on your Web Applications:
The Private folder located at /var/www/private is not writable if you don't know how to work with Linux Permissions, read my other Notes Here
The image itself is too big
If you are dealing with Sliders, sometimes they only accept jpg and not png (Also research about why you should use jpg rather than png, jpg is optimized for speed and is better than png)
Solution:
Get access to your Web Server and have SSH ready to remote connect
Identify the User Group you are operating under e.g. root, or other different user group and know your permissions
Lastly type:chgrp -R [the user group you want to have access to the folder so that the website can upload images] /var/www/private/
+1 888-688-2939 
Jack said:
Thank you