Adding shadows to simple objects in Photoshop is no big deal but adding them to irregular objects like people with awkward stances can be a bit tricky. Here I outline simply how I normally do it.
A decent hi-res image of a person against a neutral, single colour background.
I bring my image of a girl into Photoshop and cut her out of her background using the Pen tool. I’ve just started using Vector Masks so when finished tracing her outline with the Pen, right-click inside the outline and select “Create Vector Mask”. Make sure you’re not working on a background layer for this. Creating the Vector Mask will automatically cut out the object and leave the rest of the image transparent which is what we need here. The benefit of using a Vector Mask is that you can go back and adjust the Path/Shape at a later stage. Handy sometimes.
Once the girl is cut out, I duplicate her layer and rasterize it to remove the vector mask then using the Transform – Skew or Distort tool, I drag the duplicated layer to the rough angle and shape that her shadow would be, paying particular attention to the light source in the photo. Once you’re happy with that, darken this layer totally using Hue/Saturation. You might find after Transforming that the feet don’t line up properly with the shadow so using Transform – Warp, drag things back into line.
Next, add a Layer Mask to the shadow layer and apply a Black to White gradient along the direction of the shadow with black at the feet and white at the head. Next, apply some Gaussian Blur (about 10 radius depending on photo resolution) to the shadow layer and make a rectangular selection over the top half of the shadow, Feather it about 100px and apply more Gaussian Blur, this time about 30px radius. These steps give the shadow a more non linear and realistic look, fading and softening the shadow the further from the subject it is.
Finally some tidying up around the feet in the original layer where they meet an invisible floor by painting in some under-shoe shadow on a new layer with a very soft Black Brush. Change the opacity on both this and the shadow layer to about 85% to soften a little.
And yes, it is more fun to work on images of sexy girls..