Using the filepicker of steam is annoying on linux..

I suggest adding some shortcuts, for example for the users home folder, this should be very easy to implement and already help a lot when you want to share images with your friends in steam chat.

An even better solution, even if its a bit more platform specific work would be providing shortcuts for the favorite/bookmarked folders of the desktop environment / the file explorer of the system.
So that pictures / desktop / documents / download folders can also be easily accessed.
I guess there are standards on https://www.freedesktop.org/wiki/ or similar standardizing efforts.

Also add a field where the power users can type in the path they want to navigate to.

PS: Another suggestion for the chat: allow drag n' drop dropping in image fields from the systems file manager.