You're probably thinking "one cell = one pixel." In fact "one cell = one 8x8 block of pixels"; the specific pattern of colors in each 8x8 block represents the opcodes of that cell's 64-opcode program. So when the replicator takes over the map, it's changing the image to a repeating pattern of 8x8 blocks.

Now, don't quote me on this part, but I think the first replicator to take over is this (minus all the "junk DNA" between the instructions):

    [}<,]}],<}[

and the second, more efficient, one is:

    {[}<,]],<}[{

The latter means roughly:

    --head1;
    while (*head0) { *--head0 = *++head1; }
    *head0-- = *head1++;
    while (*head0) { --head1;
    // CUT HERE

which gets pasted together with whatever cell it's breeding with at the time.