XnView Software

Ok, I tried and it works. It shows a clean white color on the gif images. ... but the converting of a png from 32 to 8bit is mega slowly, when the png image is as example, some MB by filesize. And it uses a lot of my CPU in this long wait.

no, of course not. The png's are full color images (32bit ... I guess) from a screenshot program for taken copies of webpages.
You mean, the png's should be converted from 32bit to 8bit (256 colors), before to save them as gif's?
Okay, I will try it.

When I convert png images with a white background ( as example: screenshots of webpages) to the gif format (256 colors) bcoz of it's lower filesize, than it's always ends with a dirty gray instead of a clean white background. So why XnView is not able to reproduce pure white gif images? White is ...

Okay - Now it WORKS for me in v. 2.45.
I just had to rename in the PlugIns directory the webp.dll to libwebp.dll ... and after restart XnView it shows all images perfect again.

No, I just have installed since longer time the WebP_CodecSetup.exe from Google, to my Win 7 PC.
How I can update to this libwebp.dll? Coz the only one webp dll I found in the I/O Plugins of XNview's Website, is from 19. 01. 2015 !

I just wonder, that the version 2.44 works fine without any extra ...

I updated today from 2.44 to the version 2.45 and could not more view my webp images in the directories.
After reinstalling the version 2.44 all works fine again as before !
Is this a bug or there is now a special checkbox to directly activate WebP Support in XnView and I just not saw it ??