The size of your SOURCE .PNG doesn't matter because, regardless how big it is, it will be resized to a maximum of 1024x256. Obviously, it must be 4:1 aspect ratio, so yours is just fine.
After you see the error asking to open the error log, reply No to the question about seeing the error log, and YES to the question to restart Couatl. Then, before doing anything else, choose "Restart Couatl and rebuild the airport cache" and WAIT until you see the message the cache regeneration has completed.
Now you should see your custom texture available in the list of operators (or jetways, if you wanted to use for a jetway).