Create a new layer filled with white.
Open the "Filters->Distorts->Curve Bend" dialog and drag the center of the curve line down just a little as shown in the following screen shot.
Attachment:
SScurve-bend.png [ 14.63 KiB | Viewed 2122 times ]
Next, click on the "Mirror" button and then OK. This will warp your white layer into the proper shape; now we want to use that layer as a layermask for your banner.
Set your background color to black and then remove the white layer's alpha channel ("Layer->Transparency->Remove Alpha Channel")
Perform an "Edit->Cut". This will delete the white layer, but it will first be copied to your clipboard.
Add a layermask to your original layer ("Layer->Mask->Add Layer Mask..."). The default settings are fine for this.
Perform an "Edit->Paste" and then "Layer->Anchor Layer" to paste your clipboard into the layermask.
Voila!
Attachment:
FarmPic2.png [ 2.18 MiB | Viewed 2122 times ]
Note that the image itself does not actually get stretched, merely the border is changed. If you actually want to stretch the image, just apply the Curve Bend filter to the original layer (this will distort the straight rows of plants).