Fixed the if-else statement responsible for loading the correct login/register image. The SVG wouldn't work, so I just used the new avatar@2x.png, so I guess that one is staying for now.
If it works it works, I could load the vector image by changing the logo to the SVG in:
confirm-password.blade.php, forgot-password.blade.php, login.blade.php, register.blade.php, reset-password.blade.php and verify-email.blade.php.
This works good enough, so imma leave it how it is now.
As part of the new logo (https://github.com/JulianPrieber/littlelink-custom/milestone/1) I'm now using the new 'logo.svg' vector logo.
For this, I added an if-else statement in most cases, which first check for a custom logo uploaded by the admin or the avatar of an individual user. If the image doesn't exist, the default logo (logo.svg) is used.