A program called "The Gimp" is probably your best bet, but its not simple
GIMP - for Windows
Just download the link named as "installer". You should be able to use that to blend out the spot(s) but I don' think theres an automated option - from memory you have to select each point to fix by hand
I'll repeat though - its not a simple program. Originally written for Unix, it still looks like it belongs elsewhere with multiple floating panes for the command and controls