The utility 'png-text-append' inserts "text chunks" into PNG files.
*Note: this does _not_ mean 'drawing text on the image' in any way;
the appearance of the image will be unchanged.  This means inserting
an ancillary text chunk into the PNG stream, which can be used for 
example, to hold copyright information.

The utility 'png-text-dump' displays all text chunks present in a PNG 
file.

References:
http://stackoverflow.com/questions/9036152/insert-a-text-chunk-into-a-png-image