What you need:

1- One of the original nuvi 37xx logo.
2- GIR_Editor v0.30 or above
3- RGN_Tool v0.74 or above
4- updater.exe provided by Garmin.

Step 1: Replacing the image in logo37xx.bin

1- Start GIR_Editor.
2- Click the Open button and open the original "logo37xx.bin" (from link above)
3- Double click on the line "0- PNG 1024x512x32" to open the Symbol Replace dialog.
4- Click the Browse button and select the new image. This image needs to be a .PNG image with a frame resolution of 1024x512 with your image of resolution 800x480 in the top left corner. See the picture below as an example. Click Open to load the selected file, then click OK to close the Symbol Replace dialog.
5- Click the Save button and save to "my_logo37xx.bin"

The picture below shows an acceptable PNG image of 800x480 pixels in a frame of 1024x512. Optional: I use the free MS Paint on windows systems to rescale, change the frame size and save as PNG. Other image editors can be used.


Step 2: Create the .RGN file to update the GPS

1- Start RGN_Tool.
2- Enable the first firmware region checkbox on the left.
3- Click the Load button in that region on the right. Select the boot.bin file that correspond to your GPS. It is important that the HWID of your boot.bin matches the one on your GPS. Click Open.
4- Enable the second firmware region checkbox on the left.
5- Click the Load button in that region on the right. Select the new logo file you made with GIR_Editor (aka my_logo37xx.bin) Click Open.
6- Click the Save RGN button on the top of GIR_Editor. Put a filename like: "my_logo37xx hwid 1101.rgn"

Step 3: Flash the GPS with the new logo

1- Drag the new logo firmware (aka "my_logo37xx hwid 1101.rgn") over updater.exe from Garmin
2- Select "USB Device" in the "Connection Settings"
3- Put your GPS 37xx in service/pre-boot mode. See the procedure below.
4- Once the GPS is detected, click OK in updater.exe to start the programming (maintain finger on screen)
5- Once the programming is finished, click OK in updater.exe.
6- Remove USB cable.
7- Notice the GPS does not reboot by itself. No worry. The GPS is expecting more firmware that will never come. Simply turn off GPS by clicking and maintaining the GPS power button for 10 seconds until it shuts down.
8- Power-up the GPS and you have the new logo.

Put your GPS in Service/pre-boot mode

1- Turn off the GPS completely. This requires to click and maintain the power switch for 10 seconds.
2- Turn on your GPS
3- Once you see the Garmin logo, put and maintain your finger above the "G" of Garmin.
4- When you see the scrolling "System" text top-right of your finger, connect the USB cable while maintaining your finger on the screen.
5- Continue to step 3.4 above (click the OK button in updater.exe). Make sure to maintain your finger on the screen.