You're welcome...but oh that is weird.
Did you only have the image and the setup file on the USB drive? No other files? I mean if you can get it to read the USB drive to try and customize the startup screen then one would think it was set up to change that image.
It almost sounds like the you are missing something on your USB for it to react that way. Quick checklist:
1. The image file is a 16-bit color depth Bitmap with the colors setup as: R:5 G:6 B:5. If you use the images I posted above (except the mostly red background image, it's a JPG) they are images that fit that criteria.
2. The image needs to be saved on your flash drive with the .bmp file extension. For example: yourimagefile.bmp
3. You need to open a text file and put the following text in it:
Opening Customize
[FILE]yourimagefile.bmp
[VERSION]9999
~END
4. Save this text file as OpeningCustomize.txt (It doesn't say it is case sensitive but I saved it just exactly like that.)
5. Put these two files on a completely blank USB drive.
I will see if I can find out anything else on this. At least I tried to help.
Here is what mine looks like in the car. Also, when I did mine, I started with the USB drive already inserted into the radio.

I did my background too but you'll note my background is very busy? That's because, like you said, the icons cover a lot of it up. When it is on the screen, you can still see parts of it that cue you into what the background is made of without overwhelming you and without getting lost completely by the icons.
I have a feeling, the Kenwood tech you talked to either doesn't know you can change that picture or has been told not to let consumers know you can do this. Here's a test, e-mail them again and ask them how you can change the startup picture on the DDX5901HD. If they say you can't do it on that one either, then you know they're not being honest with you.