Dynamic Images General Settings
This is the base URL path that the plugin will use to intercept image requests. This can be any arbitrary path but it shouldn't collide with any paths you might be using on your site.
The signing key will sign any image URL to make it more secure. Not having a signing key would open you up to image resizing attacks, so it is obviously recommended that you have a signing key.
The default signing key is unique to your plugin installation so you typically don't need to change it.