So, the free world can use this library and the proprietary world will have to develop their own if they want to use the format. Proprietary software developers get to piggy back off of tons of free software, and then when someone decides that they don't want to enable that for their particular program/library, people complain about it. It makes no sense.
A while ago I wanted to include a library in some GPLv3 software, but I couldn't because that library was GPL2-only with no "or any later version" clause. And both versions of the GPL are not compatible unless the GPL2 software has this "or any later version" clause.
So even if you stay strictly within the free software universe, even only within in GPL universe, a strong-copyleft license is a bad choice for a library.
I think license depends on your goal. If your goal is to create a web image format.. you need all the browsers to agree to include you. All browsers will not add a GPLv3 image type. Therefore it is dead before it launched.
No, that's backwards. Microsoft and Apple and Google prevent the usage of the GPL/LGPL. Don't blame the GPL for the bad behavior of large corporations that are using their immense power to try to stop copyleft.