storeImage is a command for web scraping images. You do not need it for automated visual testing.
To create an input image for visualAssert use the SELECT button. This video explains it:
This value is the “similarity” value (also called confidence value). It tells UI Vision how similar two images need to be for visualAssert to consider them the same (0.99 means exactly the same, 0.1 means anything goes). Usually 0.8 - 0.9 is a good value for automated visual testing.