A small reminder in your admin area every time a image setting was changed (in media options, through a theme or plugin)
You most likely want to regenerate the thumbnails after this plugin reminds you to do so, if not already done I suggest you install the most used plugin Regenerate Thumbnails for this. This plugin will offer a redirection to Regenerate Thumbnails in the admins reminder notice, but you can use any other plugin you prefer to regenerate thumbnails, just click dismiss when the reminder shows up and then manually navigate to your preferred regenerate plugin.
Installation
For most webhosts the usual way:
WP Dashboard -> Plugins -> Add New -> Search `Regenerate Thumbnails Reminder` -> Install -> Activate
Manuall install:
1. Extract the zip file
2. Upload the `regenerate-thumbnails-reminder` directory to the `/wp-content/plugins/` directory
3. Activate the plugin through the ‘Plugins’ menu in WordPress