Here's how to create a simple vignette with empty background....
Create a copy of your image... click and drag the image into the create a new layer icon.
Select the background image and fill with desired webpage color.
Select duplicate image layer. CTRL+A to select the image.
Go to MENU>SELECT>MODIFY>BORDER. Border Selection is up to you. You may have to experiment to get the right look. In my example I used 5. Click OK. Don't deselect the selection.
Go to MENU>SELECT>MODIFY>FEATHER (Shift+F6). Feather radius is up to you. Mine is 5. click OK. Don't deselect.
HIT DELETE. Hide background image.
There's your vignette.
Save as png.
TIP- to add more emphasis on the border, hit delete twice after feathering the selection.
This is how the vignette image would look on the webpage...
View attachment 4201