frequency content of a signal
You can say, that upsampling (without a filter or interpolation) actually doesn't change the frequency content. The image around 0.5 fs in the higher sampling frequency signal representation is also present at 1.0 fs in the upper spectrum, but outside the presented range. However, the purpose of upsampling usually is in (almost) removing images within the nyquist range, and this implies interpolation respectively a lowpass filter.