nuvi 3x0 cannot see image files in excess of the FAT16 limit of 2GiB. I haven't checked but i have a recollection that earlier fw versions for nuvi 7x0 had the same restriction, if you aren't using V4.50 you should update it now with GarminExpress or WebUpdater, or you can get it here:
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All NA 2014.10 image file is 1.86MiB so is below the 2GiB limit as are the earlier versions of all NA which includes the 2012 on your 360. All NA 2015.30 image file is now 2.20GiB and so well above it. I can't see that bam-bam posted an image in our [Only registered and activated users can see links. ] so maybe you got it elsewhere, but as GMapTool is now reporting the image as faulty you need to replace it. For the 360 you'll definitely have to use an NA image below 2GiB for sure.