You will need,

PSP 

Graphic of your choice.

Mask: Squares sides 174 Save Mask to Desktop or a File you can find Easily.

Ok here we go.....

 1, Edit>Open files...>Open Mask You have just saved from Desktop or file.>

Minimise we will use this later..

2. Edit> Files>Open a Graphic of your Choice.

3. Edit>Copy .

4.Close original graphic.

5. Edit>Paste as>New Image 

6. Image >Resize >  Width between 300 - 350 Pixels >Lock Aspect Ratio checked.>All layers Checked. 

8. Layers> Promote Background layer. 

9. Image> Canvas size> Width> 1150  by  Height> Same height as your new resized Graphic.> Placement > Left.

10. Adjust>Sharpness> Sharpen. 

11. Effects> 3D Effects> Drop Shadow> ( NOTE: Opacity- May be reduced for a softer effect.. when using a light Graphic) Settings as..

      Vertical>0.  Horizontal>15. Opacity>100. Blur>10. Colour>Black.  Shadow on new layer>Checked

 (You are now working on new Shadow layer you. You will always be working on the right layer with out having to change to any layers yourself)

12, Effects>Image Effects>Seamless Tile...Edge>Checked. Vertical>Checked. Width>100 Transition>100

13. Layers> New raster layer.

14. Selections> Select All.

15. Edit> Paste> Paste Into Selection. 

16. Selections> Select none.

17  Adjust> Blur> Gaussian Blur. Radius>35.

18, Layers> New Mask layer> From Image. >Select from Source window  the minimised Mask> Mask Squares sides. >Source Luminance checked.

19. Layers> Merge >Merge group.

20.  Effects>Image effects> Seamless Tile>. with these Settings>Corner>Checked, Bidirectional>Checked. Both Horizontal and Vertical Offset>0, Transition%>100 Corner style Linear>Checked, 

21. Layer > Properties> Lower opacity to 65)

22. Layers> Arrange> Send to Bottom.

23. Layers> Merge>Merge All.

24. Add your Watermark.

25. File>Export>Jpeg Optimiser> Compression value set >25 

And your done!....

 ..................... 

Assemble in Incredimail>Letter Creator,

Body tab, Image >Tile

Margins>Left > Move to clear the image you have now on the left.