Just a guess, but it could be due to the fact that the image is a .png. If the black outline is actually transparent in the original image, you could be seeing an artifact of Microsoft's original .png image handler which didn't do well with .png transparency as I recall. You might try converting the image to .gif as a test.
Does the image need to rotate dynamically? You could use software on the server to handle the rotation on demand. It would be slower than JavaScript alone, but it would be more reliable.
If I understand correctly, you're creating an interactive map where the map is overlaid with an image of a car and both images get updated as needed. While you could continually update both elements from their respective servers, I'd be inclined to create a set of images of the car - each one showing the car at a different angle of rotation. You could then pre-load the set of images in the browser and simply change the 'src' property of the image with JavaScript as desired with no load on the server or the Internet connection, and no browser issues. Is that possible with your implementation?
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