Find a thumbnail in a mosaic
Mathematica - 280 247
No need to scale the target to the tile size.  The key is to match the average color for the target and for each tile.  Closest match wins.
In principle, it would be possible for the routine to choose the wrong match, but the test case suggests that this approach is surprisingly robust. The mean colors of target and selected tile were almost a perfect match: a distance of 0.00296966 away from the target mean color in RGB colorspace &quot;cube&quot; with edges of length 1.
Ungolfed 364 chars
f[{m_, tilesx_, tilesy_, targ_}] :=
 Module[{mosaic = Import[m],target = Import[targ], mDim = ImageDimensions[mosaic],
 
 (* find the average RGB colors for the target *)
 meanTargetPixelColor = Mean[Flatten[ImageData[target], 1]], m, t},
 m = ImagePartition[mosaic, {mDim[[1]]/tilesx, mDim[[2]]/tilesy}];
 
 (*Find the color distances between target mean rob and each mosaic tile 
 Select the tile that best matches the average pixel color.*)
 t = SortBy[{#, EuclideanDistance[Mean[Flatten[ImageData[#], 1]], 
 meanTargetPixelColor]} &amp; /@ Flatten[m], Last][[1, 1]];
 Reverse[Position[m, t][[1]]]]

Example
Assuming that mosaic and targethold the respective filenames.
f[{mosaic, 59, 43, target}]


{48,18}


Golfed 247
f@{p_,x_,y_,s_}:=Module[{k=Flatten,i=Import,z=i@p,w,m,t,r},r=ImageDimensions@z;
w=Mean[k[ImageData[i@s],1]];m=ImagePartition[z,{r[[1]]/x,r[[2]]/y}];
Reverse[Position[m,SortBy[{#,EuclideanDistance[Mean[k[ImageData@#,1]],w]}&amp;/@k@m,Last][[1,1]]][[1]]]]