Or...if the image is on its own layer, hide the Background by clicking on the eyeball icon in the Layers Palette. (If it's not on its own layer, ALT + Double-click on it to turn it to "Layer 0")
Then click on the Layer Styles icon at the bottom of the Layers Palette, and select Blending Options from the little pop-up menu.
By grabbing the little triangle pointer on the right side of the "This Layer" bar (near the bottom of the dialogue) and pulling it slightly toward the left, the white background of your image will drop out. The good thing about this is that it's non-destructive and you can continue adjusting the slider until you get exactly what you want....Even if you Save, Close and reopen that Photoshop document.
Click the thumbnail below for a large version:
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