Its not defined anywhere else.
We are not actually resizing an image in half here the avatar is just a thumbnail displayed at 50% of its width height etc hense the .5
The code is quite specific and you would catch your hacking issue by looking here.
Code:
Content visible to verified customers only.
Your multimedia images are limited to 50 x 37 unless of course you maybe remove the is_multimedia thing on the first line part.
In fact if your not going to resize anything down why not just comment out the whole section of code?