1. The tree view of the Select Image window is too narrow. Names in the first subfolder are already cut off.
2. The width of the column for the picture names should be reduced.
3. The size for the image preview should be increased. (I can hardly tell where I took that picture from the preview ;) )
4. Names in tree view overlap the border.
5. The mission directory should be displayed by default. I personally use loading screen pictures as steam preview also

Resolution: 1920x1080
Interface size: normal
Version: ["Arma 3","Arma3",160,136406,"Stable",true,"Windows"]