The original images taken by the monkey would most likely have been RAW files.
Slater then went through these discarding all the junk, noticed one or more that looked promising, and he would then have used Lightroom (or whatever) to work with that RAW file to create the final image. That final step probably involved cropping, sharpening, setting the colour balance exactly right, and probably some minor cloning to get rid of sensor dust spots, monkey spit etc.
The monkey pressed the button, but Slater made that image.