Images as sums of waves: the convolution theorem, what a real spectrum looks like, deleting a periodic artefact with a notch, the difference of Gaussians as a band-pass filter, and Gaussian and Laplacian pyramids.
Difference-of-Gaussians
Finding the same points in two photographs and gluing the photographs together: scale-space extrema, the 128-dimensional SIFT descriptor, the ratio test, and a homography fitted by RANSAC.