I have tried to update the currency switcher with respective flags. However during one of the session, i have uploaded the India flag as png file in the folder. Now when i change it to USD, the flag remains the same.
For Magento1 ,
In the Rwd theme currency switcher path is
For Magento2 , Default Luma theme, currency switcher path is
Try to work on your custom theme,
M1 will be ,
M2 will be ,
let me know if works or face any issue.