I added a G3 to an empty stage, then took it into composer. I selected the hand sprite & opened the sprite editor, selected a sprite and clicked preview, preview looked right, so I closed the sprite editor and the hand reverted back to the default hand. I tried this with several characters, and with different sprites, none of them would "stick".
I took the character back to the stage, thinking I could replace the sprite there, and I did this time replace it and close the editor. Then I added a talk motion it reverted back to the default again. Then I tried with the motion in place, this time it finally "stuck", but when I saved the character as a custom character with the new sprite it saves with the default again. This has been a problem for a while, so I waited for a new update and installed 3.1 recently & I thought that might fix it. Sadly, no.
I was really looking forward to the psd import,(THANKS!!) but for me that's not going to working obviously, if I can't replace sprites. BTW it works exactly the same way on my surface pro 4,which I sometimes use to draw and experiment with characters. I don't get any kind of warning message or lock up. Everything else seems to be fine, I can build sets, add cameras, add characters and motions, add voice track and lip sync, edit and render. I just can't change sprites correctly.