This is going to be a problem with pixel sprites with varying distances (scales) that don't match the native resolution of the window 1:1. This is always an issue, pixels are discrete elements, so if you have a texture and viewport that don't match, then there will always be some shimmering. Really the only solution is to use texture interpolation and mip mapping. You can do this by enabling texture filtering (probably linear) on the image, and also check mip mapping on. This will get rid of the shimmers, but add blurriness. You can then use a fullscreen shader to re-pixalize the screen to a fixed resolution. This won't look exactly as good, but it will be more stable, and you won't have problems with varying pixel sizes.
