? It takes input_image and splits it vertically in half, forming out1_image and out2_image of the left and right halves. To split horizontally in half, just change 50%x100% to 100%x50%.
Just run convert as you normally would. It appears ImageMagick tries to avoid re-compression at all costs. I tried it on an example JPG and the resulting TIFF was about the same size. Run identify -verbose your_image | grep "Compression" where your_image is your original JPG or the resulting TIFF ...