You're viewing a single thread.
My really obvious one, and a huge source of problems for me, is Discord. But the biggest one was a wild one:
Irfanview
It is a super-fast image viewer and simple image editor. Supports every format I've ever thrown at it. Bulk conversion and resize works like a charm. Hell, it's half the reason I haven't moved to Linux for my daily use.
You can probably replace it with ImageMagick.
I haven't used Irfanview in a long time now. Personally, I think there are many better alternates for it on Linux now.
Qview is open source, cross platform, and at least for me on Linux, is faster than Irfanview. I'm also a big fan of KDE's Gwenview.
If you need to do batch conversion, Converseen does the job well since it's basically just a GUI for ImageMagick