Well done, you!!!
It takes a brave "newbie" to post a photo and add that it's your best to date.
So, first off, you're not really pushing your 7D yet which means you've got a lot more fun coming your way as you learn more about it.
Second, the lens you have is not going to produce stunning shots due to its characteristics. But don't upgrade yet, as you need to learn more about the 7D first, IMHO.
Third, the "why are the dogs almost silhouettes?" question is basically to do with how cameras judge the light, and adjust to get the "right" exposure.
Your eyes do this automatically, but cameras can get it wrong quite often.
Doing test shots, then setting the aperture and shutter speed in the "M" (manual) mode will get better results. But with such a bright, back-lit scene, you'd have issues with any camera.
Cheers for now, and keep shooting!
Simon 