Consider getting that YN-560 off-camera, Strobist-style, and onto a stand with an umbrella as your diffuser (any smaller diffuser may not be as soft as you'd like). Bouncing loses you a stop or two of light, and you need everything you can get. You'd need a stand, swivel, and umbrella, and some way to trigger the flash, but the YN-560 has built-in optical slave modes, so you could use the pop-up on your D40 as the master--just make sure the body of the flash is rotated towards your pop-up flash, while the head is pointed at the leggings. A stand, swivel and umbrella would probably be in the US$60-100 range.
You may also want to back down the aperture to f/5.6 to get more light from both the flash and the ambient, since you're definitely underexposed in the original shot. Ambient is controlled by iso, aperture, and shutter speed. Flash is controlled by iso, aperture, subject-to-light distance, and flash output power. If you don't bounce the light, and you get it on a stand closer to the subject than the camera is, you'll increase the amount of light it can put into your photo a lot.
If you can get more light on the background (say, another YN-560--you could just put it on the floor in the little plastic stand it comes with--wouldn't need a second stand and swivel) and a lot more separation between the subject and the background (say 10'), that would help a lot, too. You want to light in zones where the lights don't contaminate each other. That way, you can control how bright the subject is relative to the background. The white background is generally done by lighting the background more brightly than the subject, just as a black background is done by lighting the subject more brightly than the background.