Try using a program called FotoSizer
, (you can find it here: http://www.fotosizer.com/
) It will take one or more images or a folder or folders (the loading process is not fast, but it's acceptable). The % option works best for mixed images (portrait and landscape).
For example, the 32 (5 megapixel) images uploaded to the forum's picture thread by NOTLD, which add up to a whopping 57 megs were reduced to 2.2 megs total for the 32 images, with the largest being 113Kb instead of a largest of 3Mb before the conversion. From and average of 1,836KB each, to an average of 71KB per image, and all I did was reduce them to 31% of their original size (31% was closest to 800x600 in size).
It may not be the greatest resizing program out there, but it's pretty much the easiest and most straight forward, and if all your images are either all portrait or all landscape, you can specify the dimensions.