Is data:image/png;base64 the only base64 encoding for the img tag?

Yes they are:

data:image/gif

data:image/jpg

etc...

and not only for images:

data:text/html

The format is the follow

data:[<media type>][;charset=<character set>][;base64],<data>

Se here https://en.wikipedia.org/wiki/Data_URI_scheme

and

http://dataurl.net/#about