LarBob- what machine is this? The OS Boot Screen was put into System 7.5; if you want the "Welcome to Macintosh" Screen, you need to either much around with the resource fork (PICT -> to the actual Boot Screens, there are about 4 to 8 of them) and redraw it from there or you can have a "StartUpScreen(.pict)" file in the system folder (sometimes it works, sometimes it don't - you need to play around with it). With the StartUpscreen file, you may or may not need the extension, but it has to be in 8bit color (for a color screen), it will not work with a 24bit color image.
Tanaquil - I have yet to experience the loss of a resource fork on PC to Mac Transfers. But I think I can see why it happens; if you see the blank icon, and if you double click it, then what app that would open up a blank icon file would strip the resource fork out of it. It is best to leave it alone and use a program like Utility Dog to fix the File Type/Creator ID tags first before doing anything else.
If you have a Mac with preloaded apps on it - Use UD to look at their File Type/Creator IDs and write them down so you have list of them somewhere. Unfortunately for me I committed mine to memory and... well... I got Swiss Cheese for a brain. Like you stated, this works for both apps and data files, but you need to know what those tags are.