How do I display images in 8-bit with transparancy using image list

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C#

.NET

GUI

I have been doing animations using an image list which will overlap images using transparency. Currently the images are PNG 32-bit but because there are 6 separate image lists with about 60 images each at 256x256. The render speed is slow on older machines.

The graphics in question are not photo quality and so I want to render only 8-bits to speed things up however after converting the images to 8-bit there seems to be a white outline on each image. If I select 32-bit on the image list control it is ok.

The file properties say that the image is 8-bit.

Any ideas