Failing to create realistic looking composite images in Photoshop is a natural part of the learning curve, as you discover how to use selections, layers and tools to create a believable fake.
But if you decide to share your utter failure of a Photoshop with the internet while claiming the image is legit prepare to be eaten alive by jerky commenters.
Photoshop is a powerful tool capable of outputting extremely realistic, high resolution images in any size you want, yet bad Photoshoppers somehow manage to turn the mighty Photoshop into a glorified MS Paint.
But the best thing to do when you're new to Photoshop is soldier on, continually tweaking the tools and exercising your options until you get the incredible visual results you were looking for.
See 15 Terrible Photoshops That Will Make You Laugh Every Time here