We demonstrate all-optical sensing and imaging of quasi-DC electric fields using vibrationally-resonant electric field-induced sum-frequency generation (VR-EFSFG). Two femtosecond laser pulses at fixed 1035 and tunable 3500 nm are mixed in a cubically-nonlinear CH-rich sample to which an external field is applied by means of metallic electrodes defined via electron-beam lithography. The 2D images are acquired by raster scanning the lasers in an all-reflective laser scanning microscope. Photon-counting detection is implemented in transmission mode. Volt-level potentials across sub-micrometer gaps are imaged at a rate of approximately 0.5 frames per second. Signal enhancement of up to 50 times due to CH vibrational resonance is typically obtained, as verified by wavelength tuning of the mid-IR laser in the range 2400-3400 wavenumbers. Nearly ideal quadratic dependence of the signal on quasi-DC field amplitude is obtained confirming purely cubic nonlinear interaction in the sample. Using numerical modeling we establish the connection to imaging of transmembrane potential on neuronal axons and derive sensitivity limits of the method.