An imaging system may be provided having an image sensor and a fixed-rate codec for encoding image data from the image sensor into a fixed-rate bitstream. The image sensor may include an array of image pixels with a corresponding Bayer pattern array of color filter elements. The codec may include circuits for partitioning the image data into fixed-size blocks of image data and compressing the image data in each fixed-size block based on the image content in that block using a logarithm-based quantization of selected transform coefficients. The available bits for each block may be allocated to various components such as color components of the data based on the complexity of the image content in each component. The bitstream may include header information with pointers to coefficient locations within each block. The header information may be compressed prior to insertion into the bitstream.