I am designing a music-based mobile app that will need lots of instrument graphics and sounds bytes, etc.
You own the photos, but if you are using photo of trademarked items in commercial projects then yes, you need releases.
Photographers can't use photographs of private buildings or structures in commercial work unless they get releases. They can't use automobiles without releases.
I can't see how guitars would be any different. You need a release from Fender, or any other manufacturer.
You may be able to somewhat get around this by removing any distinctive items or logos on the product/guitars. But just removing logo can be a fine line. You must make the photo of the item essentially unidentifiable as a custom item.
For example, if you were to take a full photograph of a Gibson SG, or a BC Rich Warlock, there's no way you can make those photos look like just "any generic guitar" due to their custom shapes. But if you were to take a photo of just the sound hole and strings of an acoustic, or just the fretboard of an electric (with standard inlays) anyone would be hard pressed to identify the photo as any specific model of guitar.
You definitely can't use trademarked model names to sell anything without a release first.
Amplitude paid for a license to use Fender. I doubt Fender (or Amplitube) would let you get away with doing the same for free.