First choose a movie trailer with a scene you would like as your animated gif. I have selected the flying bats from Batman Begins.
With your Quicktime movie open, and Photoshop open, go through the following steps.
1.Select a frame to start on.
2.Hit "Print Screen" on your keyboard.
3.Paste the frame into a photoshop doc the same size as your screen resolution.
4.Advance the quicktime forward a frame (right arrow key).
5.Hit "Print Screen" again and so on until you have as many frames as you like.
It will be slow at first, but after a few frames you'll get pretty fast. I only did about 20 in my animation.
Once you've screen captured and pasted all the frames you want, you should have a stack of layers like this. Select the crop tool from the toolbar, and crop the area of the video that you would like to appear in your gif. Usually your gif must be square for avatars, so hold SHIFT while you are cropping.
Further, you will want to resize your cropped image to fit the requirements of whatever forum or chat program you will be using the avatar in. Go to IMAGE > IMAGE SIZE and set the size as desired. You only need to do this once. It will affect all the layers.
Once you have done all this, your file should look something like this, with all the layers still intact.
Save your file and open it in Imageready.
This is the easiest part. With your file open in Imageready, open the animation palette (WINDOW > ANIMATION) and then click the animation options button (a little arrow on the top right of the animation palette) and then select "MAKE FRAMES FROM LAYERS." You may need to delete the first frame if you have a background frame in there.
Open the optimize palette (WINDOW > OPTIMIZE) and make sure your settings are for GIF.
Now all you have to do is save it as a gif. Select FILE > SAVE OPTIMIZED AS and save your gif file. Open it to test the animation.