In my last year of high school (for those familiar with the Indian education system, that's Class 12), for my final project I made an image compressor/decompressor in Turbo C++ that used a discrete Fourier transform (DCT, actually) to work on 8x8 px blocks of images. I based it off some stuff I found about how JPEG compression works. It worked even better than I'd hoped even though it was slow as molasses (I programmed a naive Fourier transform instead of using a FFT variant). I remember jumping with joy all around my room at 3 o'clock in the morning like an excited gas molecule in a box. Fun times.