Omitting part of Frequency domain, Fourier Transform, Image Processing In my Image and Signal Processing lecture, the Professor said that if every other column of the frequency domain of an image is zeroed out, then the reconstructed image is aliased. (along the x-axis) Is there an intuitive (or mathematical ;) ) explanation for this phenomenon?

Aliasing is "a distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal" (according to Wikipedia). The following link shows the relation between direction in the spatial and frequency domain which explains why the aliasing will be along the $x$-axis in the spatial domain.

In the spatial domain you get aliasing by undersampling an image. The effect of undersampling in the spatial domain will be overlapping signals in the Fourier domain (spacing is inversely proportional in the Fourier domain). The elimination of the columns in the Fourier domain can be seen as a type of signal distortion similar to the overlap from undersampling in the spatial domain.