There are two images being displayed when launching FSI Pages mobile. One for the horizontal orientation and one for the vertical orientation. The images are stored on your FSI Server at

/_service/FSI Pages Mobile/instructions/landscape.tif
/_service/FSI Pages Mobile/instructions/portrait.tif

You can either change this images directly or use the InstructionsImageLandscape and InstructionsImagePortrait parameters to define a new image path to the images that should be displayed.