It‘s not really hex editing, but it has the same effect.
A hex editor would display the contents of a file in hexadecimal format while imgfkr just display it as if it’s a text file.
If you’re looking for a hex editor (or learn how image files are built), I highly recommend ImHex which has a unique feature where it highlights the components of a file (e.g. the header/magic number, the metadata, the fancy number things that image files has).