On install, the installer package is designed to copy the pre-packaged icons from the module's image folder over to your Joomla! 'images/flyout' folder. If all goes as coded, this process is seemless. If you don't receive any install errors regarding the icons, you will access the pre-packaged icons in the 'images/flyout' folder moving forward. If you receive an install error with the icons you will want to manuall copy over the icons from the module package to the 'images/flyout' that you'll want to create via FTP.
Please note... the pre-packaged icons are 64x64 pixel sized. This size works quite well on most desktops, laptops and tablet displays. Once you venture to smartphone displays, these icon sizes do take-up a lot of screen real estate! If you plan to use Flyout Tabs on smartphones, you may want to use smaller sized icons. This will be totally up to you to resize the icons or find, download and use different sized icons available across the internet. It is strongly suggested to store any new icons you want to use in the 'images/flyout' folder of your Joomla! install.