PHP, 971 bytes

Draws heavily upon Yimin Rong's answer, which can be seriously golfed down, especially the array indices, and put into a Phar with gzip compression.

Download the phar

This is my improved base version at 1557 bytes, saved simply under the filename "o":

<?$X=[[99,92,45,45,97,96],[99,99,99,99,99,99],[56,80,84,84,99,85],[41,55,52,64,99,86],[32,50,59,99,87,23],[67,99,74,71,90,77],[92,99,64,64,86,66],[31,41,77,99,87,50],[92,96,62,62,99,90],[64,85,64,64,99,94],''=>[99,99,98,98,96,96],A=>[49,99,95,95,96,48],B=>[68,99,64,55,85,83],C=>[93,99,47,47,58,44],D=>[61,99,52,38,77,85],E=>[99,96,60,60,57,41],F=>[99,84,40,40,37,22],G=>[99,95,46,60,80,62],H=>[99,77,22,22,77,99],1=>[99,99,99,99,99,99],J=>[26,29,24,24,96,99],K=>[99,77,35,58,67,43],L=>[99,77,22,22,22,22],M=>[99,84,49,47,87,99],N=>[99,83,44,44,84,99],P=>[99,83,40,40,53,43],Q=>[93,91,55,57,95,99],R=>[99,84,45,65,86,57],S=>[68,97,78,78,99,74],T=>[25,25,99,99,25,25],U=>[93,84,24,24,83,99],V=>[46,88,82,80,99,48],W=>[84,99,76,73,97,93],X=>[61,99,65,73,94,56],Y=>[41,65,93,99,66,42],Z=>[63,87,99,98,86,62]];echo m(r($argv[1]),$X);function r($u){$a=[];$i=imagecreatefromstring(file_get_contents($u));$w=imagesx($i);$h=imagesy($i);$s=[];for($x=0;$x<$w;$x++){$s[$x]=0;for($y=0;$y<$h;$y++){$p=imagecolorsforindex($i,imagecolorat($i,$x,$y));if(3*$p[red]+6*$p[green]+$p[blue]<1280)$s[$x]++;}}$j=0;$k=[];for($x=0;$x<$w;$x++){if($s[$x]>$h/10)for($o=0;$o<6;$o++)$k[]=$s[$x];elseif(count($k)){$a[]=$k;$j++;$k=[];}}$b=[];foreach($a as$v){$t=[];$u=array_chunk($v,intval(count($v)/6));foreach($u as$c)$t[]=array_sum($c)/count($c);$m=99/max($t);$e=[];foreach($t as$x)$e[]=intval($x*$m+.5);$b[]=$e;}return$b;}function m($A,$X){$r='';foreach($A as$a){$s=INF;$c='';foreach($X as$k=>$x){$t=0;for($i=0;$i<6;$i++)$t+=($a[$i]-$x[$i])**2;if($s>$t){$s=$t;$c=$k;}}$r.=$c;}return$r;}

And this code was used to create the Phar:

<?php
$phar = new Phar('o.phar');
$phar->addFile('o');
$phar['o']->compress(Phar::GZ);
$phar->setStub('<?Phar::mapPhar(o.phar);include"phar://o.phar/o";__HALT_COMPILER();');