An ImageProcessor Core equivalent to the output described at https://manu.ninja/dominant-colors-for-lazy-loading-images

DominantColors.cs

        /// <summary>
        /// Returns a 1x1 pixel Base64 encoded gif from the given image containing a single dominant color. 
        /// </summary>
        /// <param name="source">The image this method extends.</param>
        /// <returns>The <see cref="string"/></returns>
        public static string ToBase64GifPixelString(this Image source)
        {
            // Leave the original image intact
            using (Image temp = new Image(source))
            {
                Bgra32 color = new OctreeQuantizer().Quantize(temp.Resize(250, 250), 1).Palette[0];

                byte[] gif = {
                0x47, 0x49, 0x46, 0x38, 0x39, 0x61, // Header
                0x01, 0x00, 0x01, 0x00, 0x80, 0x01, 0x00, // Logical Screen Descriptor
                color.R, color.G, color.B, 0x00, 0x00, 0x00, // Global Color Table
                0x2C, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, // Image Descriptor
                0x02, 0x02, 0x44, 0x01, 0x00 // Image Data
               };

                return "data:image/gif;base64," + Convert.ToBase64String(gif);
            }
        }

        /// <summary>
        /// Returns a 3x3 pixel Base64 encoded gif from the given image. 
        /// </summary>
        /// <param name="source">The image this method extends.</param>
        /// <returns>The <see cref="string"/></returns>
        public static string ToBase64GifString(this Image source)
        {
            // Leave the original image intact
            using (Image temp = new Image(source))
            using (MemoryStream stream = new MemoryStream())
            {
                temp.Resize(3, 3).SaveAsGif(stream);
                stream.Flush();
                return "data:image/gif;base64," + Convert.ToBase64String(stream.ToArray());
            }
        }