Change grid.jpg; update browse.php; update login.php and register.php

Change grid.jpg:
The old image looks not too good with the style of the site.
The new image is from and subtlepatterns.com and licensed under the CC BY-SA 3.0.

Update browse.php:
Making it actually work. I don't know why the changes are being display that ugly...

Update login.php and register.php:
Username, password or email field shouldn't be empty! Something is wrong with my formatting... :(