I don't really know what you are saying because:
I'm not using megapov, I'm using pov-ray 3.6
My error has to do with opening the tga image
I'm not good at pov-ray so I don't know what you mean by make the lengths searchable and replacable.
Here's the exact file I'm using, can you edit it if you find something wrong.
/**********************************************************
Include file for Toribash PovRay rexporter (http://www.toribash.com)
By Nokturnal 2006
Modified by Anthony Cox 2006
To use this file; change the include directive in screenshot.pov to "identify.inc"
Below are some modifiable settings to change how the players look.
***********************************************************/
// Player A's (red in-game) colour.
#declare playerAr=0.8;
#declare playerAg=0.5;
#declare playerAb=0.3;
// Player B's (blue in-game) colour.
#declare playerBr=0.3;
#declare playerBg=0.5;
#declare playerBb=0.8;
// Use a head skin and name of skin (takes from the heads folder).
// Player A.
#declare playerAskin=1;
#declare playerAname="slainvet.tga";
// Player B.
#declare playerBskin=1;
#declare playerBname="uke.tga";
// heads directory (must include trailing slash ie "dir\\" or "dir/").
#declare headdir="../heads/";
// Are the skinned head reflective (light areas are reflective)
#declare reflectiveheads=1;
/*********************/
// //
// END OF SETTINGS //
// //
/*********************/