I haven't really tried xnconvert, it was just an example that is free for commercial use and supports webp.
It seems there's a bug in it and it ignores the "preserve extension" option. I expected it to work. Well, maybe there's a workaround somewhere. Can't really search for it now.
I've used Irfanview for my personal use, but you'd have to buy a licence for commercial use... if you want to be honest and respectful, of course
There's also
@bossapplesauce's cruncher in the tools section of the forum. I'm not sure if the transparency issue is resolved by now, though.
Well, there's always imagemagick and scripting...
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