Fallback for variables
You can have fallbacks for your variables, in case they wouldn't contain anything inside them. This is the code you can use:
{fallback(image/1,"http://xy.com/image.jpg")}
so choose the variable you want to give a fallback to, like in this case:
{image/1}
and copy the middle part out, then write it manually into this codesample, and separate the fallback with a comma.
You could have more fallbacks too, just put one inside another:
{fallback(fallback(image/1,thumbnail/1),"http://xy.com/image.jpg")}
This means, that if your {image/1} variable doesn't have any data inside it, the {thumbnail/1} will be used, but if the {thumbnail/1} doesn't have anything either, the http://xy.com/image.jpg will be used in the input you wrote this.