Remove unwanted background detail from images in Adobe Photoshop (CS2/CS3/CS4/CS5)
Layer blending allows you to control the range of light or dark pixel values in the active layer and in underlying visible layers that appear in your final image. Among other things, you can use this feature to remove a dark background from a light image or vice versa.
To remove a range of dark pixels from the background layer, open an image in Photoshop. In the Layers panel, convert the background layer to a working layer by double-clicking on it and clicking OK in the resulting New Layer dialog box. Next, click the Add A Layer Style button at the base of the Layers panel and choosing Blending Options from the resulting pop-up menu. On the Blending Options panel, choose Gray from the Blend If pop-up menu to specify a blending range for all channels. Finally, move the black triangle on the This Layer slider to the right and watch the background disappear! Pixels with brightness values less than your specified setting remain unblended and are excluded from the final image. This, however, is creating a blunt edge between dark and light pixels.
To create a smoother blend, start by [option]-dragging ([Alt]-dragging in Windows) on the This Layer black slider to split it in half. Then, drag the left half of the slider to 80 and click OK.