With patch applied, and compiled with ADD_IMG_ALT_MODIFIER
can use -alt-text <text> option.
Modifiers available in the <text> -
%f add full path/file name of src.
%n add file name only, no path.
%p add just path name, if any.
%s add file short title, no extension, or path.
%x add file extension only.
Following 2 only available if source image file found
using a relative path. Will not function if URL used
in 'src' attribute.
%t add the date and time of file, if file found.
%z add the size of file, if file found.
Of course, if you want to add a '%' to the text, just add 2.