There are two ways to do this. You can use billboards or you can set the color of the background and the background image from the script. I prefer to do it from the script.
Enable( FULLWINDOWMODE );
SetColor(BACKGROUND,RGB (2, 40, 80));
Enable( BACKGROUND );//This is optinal. Try without it
szImagePath = "c:\\images\\picture.bmp"
It would be better to store the image in the binary table and then stream it out to the temporary directory, which MSI creates at the start of the install. Then you could use szImagePath= SUPPORTDIR ^ "picture.bmp"
This isn't very easy so you probably don't want to try this right away if you are a beginner. The function for streaming binaries is not built in, but it can be found from InstallShield's examples folder or some similar directory.
Billboards are probably a better solution for a beginner:
Select Billboards from the view list. It's located under the topic 4.Design the user interface. Right click on the Billboard text in the window to the right of the view list.
Select new billboard. Then you must add an image that will be shown during file copy. You can enter text and select colors for the billboard from here. Then just compile and run your project.
I have had some nasty problems with billboards and setting the background properties from the script. If you use fullscreen background focus seems to disappear from Windows Installer dialogs. It can be a VERY annoying problem. I haven't yet been able to resolve this.