I too am having issues with the insert image in the splash screen widget, I am using Chrome Version 66.0.3359.181 (Official Build) (64-bit) on a Windows 10 OS, it will not work in IE or Firefox either, however, if I test on a machine with a Windows 7 OS it works fine. Any help would be greatly appreciated. I have tested this on versions 2.5-2.8 of WAB
Kate