Using "Pucker Tool" in Photoshop may make the fat body of an image to look slimmer. But, still not really easy if you are a beginner in Photoshop. You can try searching for "Photoshop body thinning" in google or youtube.Actually, there's an easy way to look slimmer in a photo without Photoshop or even without any photo editing software. Here's a link that helps you to create a photo of slim body. After clicking the link, you'll be able to create a photo of your weight-loss body, by just adjusting the weight, height and desired weight loss/gain.
Have fun slimming now with WeightMirror!