Hey Whammy,
I recreated your issue by just painting over the original splash screen inside of gimp and exporting it as a .BMP. Just that turned the screen blue.
There is a feature request in now for the options like windows has where you can simply load it in through the project settings in UE4.
I will also open a bug in the fact that certain .BMP formats are not supported on Linux.
Thanks for reporting this,