If you specify the output -Path as "something.jpg" Export-Bitmap always exports as .jpeg not .jpg
 
[56]: export-bitmap .\cold_welcome_1440x900.jpg background.jpg
[57]: ls .jp

Directory: Microsoft.PowerShell.Core\FileSystem::C:\Users\Joel\Downloads

 
Mode LastWriteTime Length Name

---

-a--- 8/24/2009 3:44 PM 133422 background.jpeg
-a--- 8/24/2009 3:13 PM 624502 cold_welcome_1440x900.jpg