video letterboxing in premiere pro with the same image as background
Copy the picture into a second video track. Then, zoom in the picture on the lower track (i.e. increase it's size in the effect settings panel). Then, apply a gaussian blur from the effects panel and raise the blur factor in the effect settings panel until you are satisfied with the result.

If you need more detailed instructions, please provide some additional info on how you set up your project in Premiere Pro, what you have tried to achieve this and why it didn't work.