XOR two monochrome images
The Fx Expression Language (ImageMagick), 8 4 bytes
EDITS

Simplified to u!=v, -4 bytes

As &quot;Fx Expression Language&quot; is apparently Turing complete, I've re-profiled my answer to it (was Unix Shell + Image Magick).
Golfed
u!=v

Fx does not support bitwise XOR nor bitwise NOT, so I've used != instead (which works just fine for the pure BW images).
u=&gt; stands for &quot;first image in list&quot;
v=&gt; &quot;second image in list&quot;

Input and output are implicit (controlled by the interpreter).
Usage
ImageMagick convert utility, serves as the &quot;Fx Expression Language&quot; interpreter, when invoked with -fx, as illustrated below:
convert $1 $2 -fx u!=v $3

The arguments are:

Input image A
Input image B
Output image O (A^B).

Sample output
convert a.png b.png -fx u!=v o.png