Location of image to replace:
/System/Library/CoreServices/SystemStarter/QuartzDisplay.bundle/Resources
Image Name: BootPanel.pdf
Image Type: PDF
Image Size: 472 x 360

From the Finder click the "Go" menu and select "Go to Folder..." Keyboard shortcut is Apple+Shift+G.

Enter this path or use copy and paste:
/System/Library/CoreServices/SystemStarter/QuartzDisplay.bundle/Resources

Work on a Backup!
Copy the BootPanel.pdf out of the Resources folder to your desktop. Rename it BootPanel.pdf-off or BootPanel.pdf-backup or anything like that. Drag it back into the Resources folder. You will be asked to Authenticate, enter your password you use to login with. Make another copy of the BootPanel.pdf to your desktop, this is the file you will modify.

Using the BootPanel.pdf you copied to the desktop as a template, open it in Photoshop or other graphic editor of choice. Make your changes, be sure to merge all layers and save it as "BootPanel.pdf". Your custom image will not work unless it is saved as a PDF; saving it as a TIFF, JPEG or any other format will not work!

Once you complete your edits drag the BootPanel.pdf from your desktop back to the Resources folder. You will be asked to Authenticate again, you may be asked to enter your password again...

Your Boot Panel has now been replaced. If you followed all the steps correctly you should see your new Boot Panel image the next time you restart. If you don't, you may have missed something in the instructions.