First sure open image
Now press the "Jump to Imageready" button, alternatively you can press CTRL+SHIFT+M.
Ok now you will see your two layers in Imageready. At the bottom you will see the Frames area, this is what we will be using to create our blend between images. Firstly select the "Duplicates current frame" button.
Now you want to goto over to your layers and press the eye icon to make one of them invisible, so you have two seperate images on the two frames.
Now create a third frame, and make it so that it shows the same as frame one.
Now we need to use the Tween tool I explained earlier. The tween tool is located directly left of the "Duplicate Current Frame" button.
Click onto frame two, and then choose the tween tool. This will bring up a menu. Its upto you what settings you use, the more frames you have the bigger filesize the end result will be. I'm going to go for three frames.
As you can see this has added three frames between frame one and frame three, each one slowly blending into the next. Now, move to your end frame and repeat the same tweening steps again. This should give you the same results as me.
You can now press the Play icon (two across to the left of the tween icon) to view your animation upto now. With the hard work now being done, we can simply choose to change how long each frame stays visible. Its usually a good idea to not change the time of the frames that are in the "blending stages" as this will result in a choppy animation. You can change the time of each frame by pressing the little arrow next to where it says "0secs" and clicking on your desired time. I have choosen a two second pause for each of my images once the blending has finished as you can see below.
Once your happy with your finished product, you save it by going to File, Save Optimized As (remember as a .gif), and then choosing a file name and place to save your new animation.
For full article and better resolution image, Visit http://www.tutiki.org/w/index.php/Basics_Of_Animation