For Skin imaging, high spatial resolution is important. It would be ideal to use a broadband high frequency (based on the required resolution) US probe and a data acquisition system with required sampling rate (at least meeting the Nyquist criteria). If you are using single element probe, then you may need a good scanning setup also.
Imaging Speed, especially for the human imaging experiment. As noted by othter people, the transducer had better be have high central frequency( 20MHz~50MHz) and broad bandwidth if you want to acquire high spatial-resolution vessel image.