1. Install Quicksilver. Believe me, you will not regret installing this.
  2. Click this link to install image manipulation plugin
  3. Configure your quicksilver trigger key. I prefer F1.
  4. Select an image file in finder
  5. Press Trigger Key (e.g. F1)
  6. Type in SC, I think QS will map it to scale image w/o even typing the A in SCALE, press tab
  7. Type in the percentage you want the resultant image to be resized. You may type in percentage e.g. "50%" or Width e.g. "100" and press enter.
  8. Nearly an instant resized image file is located in the same folder with the file name "xxx copy.jpg"
  9. Particularly useful for Batch conversion of multiple files. (select multiple files in step 4)
  10. For the "SC" command, you can replace with "FO" and tab and enter the file format (e.g. JPG) for instant image format conversion.
  11. If you installed Flickr Plugin for Quicksilver, you can even upload an image file to your Flickr account.
  12. For instant hacking QuickSilver, please refer to this cheat sheet.