mw4kids:
I thought it was the admin/images/logo.gif file, so I deleted this image file ...
By deleting the logo file you leave yourself open to the ALT text being displayed in place of the missing image file, on Firefox anyway.
It would be MUCH smarter to just replace the logo.gif file with a blank.gif or some other single-white-pixel image file.
If you just replace the logo you don't want to display, with an empty image (single-pixel white), you can easily add a real logo image later simply by replacing the logo file contents at such time as you have something to put there.
Editing the /admin/invoice.php file means you'll need to edit the file again later, and thus you'll be coming back asking how to put it back to normal again.
Replacing the image as required is much simpler and logical.
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