This isn't really an extension, but:

Using rsizr.com:
Step 1.
Select an image.

Step 2.
Go to rsizr.com.

Step 3.
Select "Upload an image". Browse to the file you want to retarget. Click "Ok".

Step 4.
Using the Preserve tool, select all of the details that need to keep their aspect ratio (Jack-O-Lantern).

Using the Remove tool, select all of the details that you want to remove (Tree, or maybe a person).

Made a mistake? Don't worry, just click "Erase" to remove the mistakes you made.

Step 5.
Select the retarget tool.

With the mouse, grab the arrows at the sides of the image (One on top, one on the left), and drag them as far as you want to shrink the picture. Don't want to shrink it? Do it anyway.

Wait for the red bars to flash through your picture.

Step 6.
Grab the squares on the corners and sides of your picture, and retarget!

Step 7.
Hit the save button on the top bar. A dialog box will appear to the right of your photo. Choose your settings, and click on "Save".

Step 8.
Enjoy your photo!