Only text data is included in the project I downloaded on Github.

The png file is registered as "Git LFS", which is why it does not open well if you simply download it as a ZIP file.
(I'm not familiar with Git LFS yet, but it seems that only the information that the file is there is written in it, not the actual file.)

Issue was listed as a problem with Git LFS.
"Download zip" button doesn't get lfs files#903

As a countermeasure, the repository could be opened successfully by retrieving it locally with the git clone of the git command.

If you can use git, try using the git command as described above, or ask the author if you can manage it with Git LFS disabled because the repository doesn't seem to have any large files registered.
(If you look at the name, is it you...?)