Harrison try this....
Lets say the pic you want to use is 500 width x 900 length
and the siggy size is 200 w x 600 l
I first make a 'New' page set to the siggy size. Then I copy and paste the pic onto the siggy. Now the ends will be cropped but the whole pic is there. Go to 'Layer' and choose "Layer size' towards the bottom. A window will pop up that you can change the size but....make sure the link is locked on the side of the width/length windows.
Now if the pic ends up larger you can move it around until you got what you want, then under 'Image" choose "Crop to Selection" and it fits.
if smaller, you can play with fill colors or clipping small pieces of the original pic and layering them to stretch.
If you look at my sig, the cliffs of Dover are actually the same - I just cut and paste several sections and stretched it across.