definitely GIMP you can find some info here: http://www.gimp.org/windows/ Now i use the image editing software PhotoStudio, it have bother versions as mac photo editor and win photo editor. Functions as remove image background, add photo effects, clear blurry photos and so forth are all allowed.
well, PowerPoint is the known slide show software for win, powerful in words slideshow but not that stunning in photo slideshow. Well, if you're doing photo slideshow for public presentation, then I suggest you to make videos from photos. This ensure you everything will be OK and you have nothing to worry about. How to make video? Google and you will find tons of tutorials with help of video editing program.\.