Попробуйте это:
using System.Drawing;
using System.Drawing.Imaging;
Image gifImg = Image.FromFile(pathToGifFile);
FrameDimension dimension = new FrameDimension(gifImg.FrameDimensionsList[0]);
// Number of frames
int frameCount = gifImg.GetFrameCount(dimension);
// Return an Image at a certain index
gifImg.SelectActiveFrame(dimension, index);