For free graphics manipulation apps, try Gimp http://www.gimp.org/macintosh/ although you will need to install X11 first from the Tiger disk to make it run, or Image Tricks (much more basic but a bit of a resource hog) - http://www.belight soft.com/download/download.php .
Others will come along with further choices, try them all and use what suits you best.
I think preview also does it, if not use GIMP as 'hugtheslug' said or use a very simple image resizing program 'Image Well' which can be found here - http://xtralean.com/IWOverview.html
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