We get the following image and from the name of the challenge we are probably going to use binwalk.
The file and exiftool don’t give us any hints so we just move to the binwalk command where we find that there is another a zlib file and png image embedded.
$ binwalk PurpleThing.png
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 PNG image, 780 x 720, 8-bit/color RGBA, non-interlaced
41 0x29 Zlib compressed data, best compression
153493 0x25795 PNG image, 802 x 118, 8-bit/color RGBA, non-interlaced
After spending a lot of time trying to find anything useful in the zlib compressed file nothing seemed to give me any hints so I just moved to the png file. This was a bit tricky to extract and it can be done with some flags of the binwalk tool but I just went on and used dd since it was much easier.
$ dd if=PurpleThing.png of=out bs=1 skip=153493
11309+0 records in
11309+0 records out
11309 bytes (11 kB, 11 KiB) copied, 0.410729 s, 27.5 kB/s
The dd command took as input (if) the png file and produced the out file as output, also I instructed it to skip the first 153493 bits and output the rest.
The out file was indeed a png file and by opening the images we successfully find the flag ABCTF{b1nw4lk_is_us3ful}
PS. It is worth mentioning that the a zlib compress file found can be found in normal in png images since it is a way to compress the data of an actual image.