The file you're looking for is just a batch file that replaces the existing "ssfloat.bmp" file with your custom one.
All the details about this, including resolution and transparency and the batch files, are in RA Knowledgebase
Article ID # 28335 (TechConnect required).
An excerpt from that Knowledgebase article:
Changing the Default Screen Saver Bitmap to a Custom Bitmap
Create a file on a desktop or laptop computer named autorun.bat.
In the file, place the following three lines, which vary according to the device you're using:
If Using a Compact Flash Card
copy \"Storage Card2"\oem.bmp \Windows\ssfloat.bmp
copy \"Storage Card2"\oem.bmp \"Storage Card"\Windows\ssfloat.bmp
pause
If Using a USB Memory Stick
copy \"USB Storage"\oem.bmp \Windows\ssfloat.bmp
copy \"USB Storage"\oem.bmp \"Storage Card"\Windows\ssfloat.bmp
pause
Replace oem.bmp with the name of the custom bitmap.
Place the file autorun.bat on a Compact Flash card.
Insert Compact Flash card into the external slot on the terminal (if using a USB memory stick instead, the autorun.bat file will not run automatically).
Make sure the files are copied correctly by observing the Copied 1 file(s) message.
After completing the procedure, close the window by pressing a key or tapping on the X in the upper right.
The autorun.bat will not fire on insertion of the USB. Manually cycle power with the USB storage device in the slot to fire the autorun.bat.
NOTE: For PV+ 400 and 600 units, reset the terminal so the bitmap will be properly loaded into memory.
