When logging into your imageshack account you can enter either your username or email in the first field, followed by your password in the second.
If you forget your password click the link to learn how to reset it.
There is also an option to keep you logged into your imageshack account. This is not recommend unless using a non public computer. If you close your browser before logging out, you will automatically be logged in on your next visit.
To log out of your account click the dropdown arrow at the top right of the screen, next to your username. From the menu select "Logout."