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.
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?