Hello Arash Emami,
Currently we don't support Preview image. We'll consider to implement it for the next releases. You can track and vote for new features here
. (Your request will be available there in a day).
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.