Do you notice that part of my image is peeking out all around the heart?  Here's what you do.  Use the mover tool to make the heart the active layer, or click on that layer in the Layer Palette.  Now use the magic wand to select around the heart.  You do this by clicking on the background area around the heart.  Leave this selected.  Switch back to the Image layer making it the active layer.  Hit delete on the keyboard.  This will remove all the parts of the image that were around the outside of the heart.

If you find that you still have a problem with the image showing up in any way outside the heart, this is what you can do.  While the image layer is still selected, go to Selections/Modify/Expand, and expand 1 pixel.  Then hit delete on the keyboard.

Now for the base.  I had already selected just one of the bases from the set and copied it as a new image.  Before adding it as a new layer on my heart, I first use the magic wand set at a tolerance of 30 to delete the drop shadow from the base.  Look carefully at the Tool Options window.  Please make sure your Tolerance is set to 30 and Sample Merged is NOT checked.

Now copy and paste the base as a new layer.  Name this layer Base.  Move it under the heart layer if you want to have the heart sit on another section of the base like I did.

You should now make the choice of which solid color would be best to fill your Background layer with.  This is a very hard choice.  You can't choose any color that might be in the heart or the image in the heart because that would make those parts go transparent also.  But, you can't choose some really wild color that might be picked up by the edges of your heart and show up badly on the page you put your heart on.  Of course if the heart is going on a page with a white background, it doesn't matter if there is any little specks of white around the heart.  But how would that look on a page with a black background?  In my opinion, it's best if you know the exact color or background your transparent heart is going to go on.  Then you can match that color as your background color for the animation.  For instance, if my mouse heart was going on a blue background, I would choose blue, the same shade blue if possible, as my solid background color.  If you just don't know, try a neutral shade of gray as the solid color to fill your background layer with.