Sounds like your database is hosted over the network. Even back when FileMaker 6 was state of the art, changes made to a global field were not retained when the change is made from the client of a hosted database.
You'll need to do one of the following:
Take the file down off the server and open it directly with FileMaker Pro. Then make your changes to this global field.
Insert the logo into a non global field and Use perform script on server to copy the new logo into the global field.
Insert the logo into a non global field and Use a server scheduled script to copy the logo into the global field.
Insert the logo into a non-global field and use a script set to run "OnFirstWIndowOpen" (See File Options) to copy the logo from the non global field to the global field.
Use a non-global field in a single record table to store the logo and use relationships with the X operator to make the image accessible on any layouts where you need to show the logo.