Removing White Backgrounds
Open the image with the white background. Check the color depth (Image || Increase color depth) and make sure the image is 16 million color depth. If not, select this option.
In the top menu, click Layers || Promote Background Layer (or right click on the layer and choose Promote Background to Layer). Jpegs have backgrounds and cannot become transparent. Gifs, however can have their white backgrounds removed but not until the background is converted to a layer.
Choose the magic wand tool . You will have to play with the tolerance depending on the gif you are working with. Click on the white background to see if some of your image is selected with the background. Left click on the image to remove the marching ants. Play with the tolerance until you have just selected the background.
When you have just the white background selected, click the delete key to erase the white background,.