How can I change my billboard image in interface?

Create a constant referencing your image and use that constant in the interface in a call to a!documentImage().

can you tell me the expression I dont know the exact expression for it ?

Have a look at this pattern for inspiration. In line 26, use a!documentImage(cons!YOUR_IMAGE) instead of a!webImage()

docs.appian.com/.../cards-as-list-items-pattern.html

 a!billboardLayout(
            backgroundMedia: a!documentImage(
              document: cons!WR_NITIN
            ),
            backgroundColor: "#f0f0f0",
            overlay: a!barOverlay(
              contents: {
                a!richTextDisplayField(
                  labelPosition: "COLLAPSED",
                  value: {
                    "TEVA`S   Hotel"
                  }
                )
              }
            )
          )

Hello Rahul, you can see the code I have sent,

In these first you have to create a document from new objects and then there is a checkbox click on it than it creates a constant automatically and then inherit that constant in your billboard layout.

Hello Nitin, It is showing error 

Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function a!billboardLayout [line 8]: A billboard layout has an invalid value for "backgroundMedia". "backgroundMedia" must be null or a document image, web image, or web video. Received: BillboardLayout.

What I have written in expression:

a!billboardLayout(
backgroundMedia: a!documentImage(
document: cons!ET_WHAT_IS_A_TIMESHEET
),
backgroundColor: "#f0f0f0",
overlay: a!barOverlay(
contents: {
a!richTextDisplayField(
labelPosition: "COLLAPSED",
value: {
"TimeSheet Submission"
}
)
}
)
)

(I have created the constant referenced it to my image)

Always use these for sending the whole code for the best practice

sure thanks.

Create like a new document and save it. Then after there is a constant automatically create and use these constant in your billboard

a!billboardLayout(
  backgroundMedia: a!documentImage(
    document: /*cons!ET_WHAT_IS_A_TIMESHEET*/
    /*use your constant which is contain your image*/
    cons!WR_SCREENSHOT_106
  ),
  backgroundColor: "#f0f0f0",
  overlay: a!barOverlay(
    contents: {
      a!richTextDisplayField(
        labelPosition: "COLLAPSED",
        value: {
          "TimeSheet Submission"
        }
      )
    }
  )
)

It works .