After upgrading to Windows 10 a while back, I noticed that some of the taskbar icons started to look funny and had a white background instead of being transparent like the others. While this UI problem doesn't cause issues with launching or running the program, it is an annoying eyesore.
I decided one day to try and figure out the cause of it and spent several hours trying to hunt and track down the fix, only to be left with countless websites talking about clearing the thumbnail cache etc, none of which worked. Somewhere in the search quest, I ran across a post where it was stated that SageThumbs was the culprit. I use SageThumbs for displaying various image formats as a thumbnail which isn't native to windows. The author of the post said that he simply uninstalled SageThumbs and everything went back to normal. I tried this, restarted and nothing happened. No dice, so I dug deeper. That's when I ran across an issue with the program where the developer said it was a known issue with the .png image format. (All of the icons in Windows 10 are .png files) His suggestion was to "disable .png" in the SageThumbs settings. I headed off and disabled the .png format, restarted and nothing. I tried repairing SageThumbs and nothing. I tried clearing the thumbnail cache and restarting. Nothing. Nothing I tried, would fix the problem with the stupid white background of the taskbar icons.
I even noticed that the context menus when you right click were showing up with a white background. Along the way, I found a comment where someone posted that changing "Use Small Icons" in the Settings -> Personalization -> Taskbar would remove the white background. I gave this a try and sure enough, switching to small icons would remove the white background, but I didn't want small icons. I finally gave up and decided: "Whatever, I know what the culprit is, so I'll just live with it because the thumbnail icons are far more valuable to me than a pretty taskbar icon."
A few days later, I forget why, but I ended up doing a search on "sage thumbs messes up windows 10 taskbar icons" which led me to the reddit post Do not install Sagethumb on Windows 10. It practically breaks windows, taskbar icons, apps and all. I looked through the post and while there wasn't a solid fix, I did manage to piece together a few of the fixes suggested and have FINALLY figured out how to fix it once-and-for-all as a solid fix, without having to completely re-install windows. (Yes, that was the solid fix that kept coming up when digging around the internet)
So, the steps to fix the SageThumbs problem are actually quite simple:
You can restart explorer several ways: Reboot, Sign out and Sign back in or Kill and restart explorer. I'll explain how to restart explorer but if restarting the computer is easier for you, just go that route.
taskkill /im explorer.exe /f && explorer
This will make all of your icons disappear for a split second and they should all re-appear fixed with no white background!
I was able to replicate the problem and fix it on multiple tries, so I do know it is for sure, related to the .png in SageThumbs and using the Disk Cleanup to clean the thumbnails is the guaranteed fix. Hope it works for you!
As of 2017-05-09, there was an update to SageThumbs to version 22.214.171.124. I installed it and it worked great until the 1703 creator update decided to finally roll out on my computer. It all went back to the white background. Looks like the issue still isn't fixed with the new update :-(