First it's cropped it into a square.
Then it's reduced to a 30x30 pixel grid...
...and converted to grayscale.
The image is further limited to just 21 shades of gray.
Each of the 21 shades is assigned a codon, a group of three nucleotides (some combination of adenine, thymine, guanine, and cytosine—A, T, G, and C for short). For a 30x30 pixel image, a synthetic sequence of 900 codons made up of 2,700 nucleotides is needed.
The researchers didn't line up the codons back to back like this; instead, they were dispersed in short chunks of DNA throughout the bacterial genome. They also inserted special nucleotide sequences for error checking, as well as sequences to flag what's a pixel rather than the host's DNA.