how can i modify the login html? I specifically want to add an image below the title.
You can add a logo to the login page by uploading one to Settings → General Login Page Logo.
If you need further tweaking, you could use the Control Panel CSS plugin.
I wouldn't recommend modifying Craft's templates.
I solved this by adding a module that is supposed to act as a sitemodule with different functions I use over most of my projects.
Inside that module you register an asset bundle that ultimately adds a CSS stylesheet to the controlpanel.
This way you can alter the look of the loginpage without tampering with any of Crafts innards. As can be seen from the screenshot, I have hidden the title and added another background image.