Hi Adyan,
Most of us know there is something called exposure, which is the process of ensuring the right amount of light gets into the camera. Too much light into the camera and the photograph will be washed out; not let enough light and the photo will be too dark.
In this photo, not enough light got into the camera. It's likely this happened because the camera’s light-meter was fooled by the white sheets into thinking the background was brighter than it actually was.
This is a common fault with all cameras when confronted with lots of white in a scene. For this reason, most experienced photographers will actually use manual exposure mode when photographing tricky light situations. Another option is to use the Exposure Compensation dial on the camera to add more exposure. In this instance a +1.0 or even +2.0 compensation would have made a large difference to this picture. (Note: One thing to remember when using Exposure Compensation is to reset the dial back to “0” when you have finished making photos in that situation!)
Now, how to improve this image? The simple option is to go into Photoshop or a similar app. Rather than making an overall adjustment to the exposure of this image though… just select the dog and make that part of the picture lighter. The darker sheets will help draw our attention into the centre of the photo.
Use the Lasso Tool with a Feather setting of about 20 pixels (experiment to suit the image) to select the dog. Once the selection has been made, use the Curves tool to lighten the mid-tones of the picture. (See the example in the comments section below.)
Even if you do get the overall exposure correct the majority of the time, this simple technique is a very good "get out of jail card".
Cheers for now,
Anthony.