[Urendi Maleldil]

You could do the bulk of the resizizing with Photoshop. There's a macro recorder in there somewhere you can use to apply the same settings to multiple files. I used it to change the color of the Cheaters race so they were gold instead of the Default silver.

I don't remember where it is, but I found it with the Photoshop help file. Afterwards you can do the less-repetitive retouching file by file.