There are two files used
1. newposts.gif
2.nonewposts.gif
A set for each style installed.
There are a couple ways to do this like creating a new style and adding the image folders. The best way to explain this would be to try a custom style already made like the black and gold Rick did.
HERE It is a custom style with its own images.
In simple terms replace each one with your new graphic named the same as above.
Depending on the style used is where your graphic is placed.
The stock install has two locations
/images/general/default_dark/
/images/general/default/
not to mislead you but this method will replace the graphic across all categories. So you can't have a different graphic for each category.
