No specific dimensions and naming convention is given by Appcelerator for splash screen landscape mode support for iphone 4/4s, iPhone 5/5C/5S and iPhone 6.
Dimensions and filenames are provided for iPhone 6+ devices to for splash screen support in landscape mode and its working.
Please find the below link,
I have created the new project in Appcelerator which provides default set of images used for App icons and splash images. While running this project the splash screen opens in portrait mode only in iPhone 4/4s, 5/ 5s/ 5c and 6.