dpoakaspine

Disallows PNG files. Usefor for multi-user environments with restrictive file settings and performance oriented sites.

Disallow PNG

Disallows PNG files. Usefor for multi-user environments with restrictive file settings and performance oriented sites.

tpaksu

This plugin uses CSS technique to fix the png image transparency issue and jQuery to apply this script to all of png images shown in the page. 
This doesn’t work for positioned and tiled png background images. Maybe in next versions.

WP pngfix

Plugin that fixes the png image transparency issue on IE.

Disallow PNG vs WP pngfix