Do you ever heard of PLACEHOLD.IT ?
What’s PLACEHOLD.IT ? A quick and simple image placeholder service. How does it work?
Just put your image size after our URL and you’ll get a placeholder.

Like this:
You can also use it in your code, like this:
<img src="">
You will get a image placeholder like below:
Placehold for 350x150

But markdown does not support defining the height and width for the image.
so you can choose to use <image> tag if need. Is it very simple and useful? Have fun!