Select the new image you created, Go to Edit and Paste.Open the PNG image in separate window, Go to Edit and Copy.Create a new image, the same size as the PNG, with the Color Space set to RGB (under Advanced Options), and click OK.Start GIMP ( If you are not using GIMP and have a DDS plugin for Photoshop, skip to the 'For Photoshop' Section).Save as a PNG (don't worry about setting transparency if you are unsure how to do this).For the purposes of these instructions, if you are using Photoshop go to File->New->.
NOTE: If you have a DDS plugin for Photoshop, then you skip using GIMP altogether - to download that plugin for Photoshop go here.