Power Zoomer plugin by xmlswf allows you to show big images efficiently. it is highly customizable. Its compiled flash file is just around 12 KB.
Features:
Customizable zoom levels and zoom speed
Auto slide show enable/disable
Customizable icons
Customizable thumb and main image height and widths
Each product description and link to product page
Customizable font sizes and colors
Scrollable thumbs
Full screen mode
Thumbnail tooltip
Pan window
Next and previous thumbs with tooltips
Download: http://wordpress.org/extend/plugins/power-zoomer/