We created a new lead with a value for company, but not email, phone, title, or industry. We expected it to have the '1 star' image but it didn't. If you use a CASE() function, make sure it’s positioned outside the IMAGE() function.

Please check below 

CASE(  Lead_Quality_Helper__c,   1, 
IMAGE("/img/samples/stars_100.gif", "1 stars"),  2, 
IMAGE("/img/samples/stars_200.gif", "2 stars"),  3, 
IMAGE("/img/samples/stars_300.gif", "3 stars"),  4, 
IMAGE("/img/samples/stars_400.gif", "4 stars"),  5, 
IMAGE("/img/samples/stars_500.gif", "5 stars"),  0, 
IMAGE("/img/samples/stars_000.gif", "0 stars"),'' )