We demonstrate ultrafast dynamical imaging of surfaces, using a scanning tunneling microscope with a low-temperature-grown GaAs tip photoexcited by 100-fs, 800-nm pulses. We use this setup to detect picosecond transients on a coplanar stripline and demonstrate a temporal resolution (full width at half-maximum) of 1.7 ps. The temporal waveform resulting from our low-temperature-grown GaAs tip is compared with waveforms obtained by means of photoconductively gated metal tips in the same setup. By dynamically imaging the stripline we demonstrate that the local conductivity in the sample is reflected in the transient correlated current and that 20-nm spatial resolution is achievable for a 2-ps transient correlated signal. Finally, we report the characterization of a picosecond pulse propagating along the stripline for millimeter-scale distances.