If you want to use the tga's (instead of the dds files), then save the tga's uncompressed. Usually ... in the options/preferences area of your photo editor there will be a setting to turn off compression when saving tga's.

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The illusion you're getting w/ the plane not looking right on the ground is indeed due to the horizon being in the wrong place (too high or too low) in your image. It's not a function of your camera --- a spherical panorama is a spherical panorama. So it is a ground elevation/horizon issue.

The pano image codes usually have a way to set the horizon. BTW - AutoStitch is a nice code for painlessly getting panos, but I don't know if it allows the user to tweak the horizon.
http://www.cs.ubc.ca/~mbrown/autostitch/autostitch.html

Michael