Does anyone know how can I export an image of displayed part to jpg?

Have a look at the CreateImage function.

Here is a quick journal to get you started. It outputs each of the saved views to a jpeg in the specified directory.

Option Strict Off
Imports System
Imports System.IO
Imports NXOpen
Imports NXOpen.UF

Module Module1

Sub Main()

Dim theSession As Session = Session.GetSession()
Dim ufs As UFSession = UFSession.GetUFSession()
Dim workPart As Part = theSession.Parts.Work
Dim partName As String = Path.GetFileNameWithoutExtension(workPart.FullPath)

'directory to output jpegs, change as needed
Dim outputDirectory As String = "C:\temp"
Dim strPartJpg As String = ""

If Not Directory.Exists(outputDirectory) Then
MsgBox("The specified directory does not exist, journal will now exit", MsgBoxStyle.Exclamation, outputDirectory & " not found")
Exit Sub
End If

Dim wpModelingView As ModelingView
For Each wpModelingView In workPart.ModelingViews
workPart.Views.WorkView.Orient(wpModelingView.Name, View.ScaleAdjustment.Fit)
strPartJpg = partName & "_" & wpModelingView.Name & ".jpg"
strPartJpg = Path.Combine(outputDirectory, strPartJpg)
ufs.Disp.CreateImage(strPartJpg, UFDisp.ImageFormat.Jpeg, UFDisp.BackgroundColor.White)
Next

End Sub

Public Function GetUnloadOption(ByVal dummy As String) As Integer

'Unloads the image when the NX session terminates
GetUnloadOption = NXOpen.Session.LibraryUnloadOption.AtTermination

End Function

End Module

What is the code for choosing only the displayed bodies?