Today's diesel injectors operate at very high injection pressures of about 2000 bar resulting in injection velocities as high as 700 m/s in the near nozzle region. To better predict the behavior of the atomization process at such high pressures, we must record high-resolution spray images, and compare them to the numerical simulations. Now due to extremely high speed of the jet in this region even the fastest ultrafast camera fails to avoid the streaks in the spray images due to exposure times that are shorter than a microsecond but still remain too large in such conditions. Ultrafast imaging featuring ultra-short laser pulses to freeze the motion of the spray on the detector is a good solution to overcome this limitation. However, commercial high-energy ultrafast laser sources based on titanium-sapphire crystals are limited to a few KHz repetition rates. Here, we propose a method for obtaining shadowgraphs of high-pressure diesel sprays using a high-repetition rate picosecond fiber laser source for illumination and an ultrafast camera for detection. The picosecond laser consists of a homemade passively mode-locked ytterbium-doped fiber laser operating in the all-normal dispersion regime. A fiber-Bragg grating-based filter centered at 1040 nm is included inside the cavity to limit the spectral bandwidth to less than 140 pm. The laser generates nearly transform-limited pulses with 30 ps duration and less than 1 nJ energy, at a repetition rate of 8 MHz. A two-stage fiber master-oscillator power-amplifier is used to boost pulse energy to more than 300 nJ. This corresponds to more than 2.5 W of average power. Using an electro-optic modulator placed after the laser output we could easily modify the system repetition rate.