JPEGs saved directly from some digital cameras do not contain whatever data getImageSize relies on to produce an array of size and type data. Opening such an image and re-saving (even as JPEG) within an image editor (photoshop or equivalent) will 'fix' the image, allowing getImageSize() to read it.

Here's an image that causes getImageSize() to fail:
http://sitefoundry.com/php/DSC00021.JPG

Here's the same image after opening/saving; getImageSize() copes fine with this one:
http://sitefoundry.com/php/DSC00021fixed.jpg

Here's the output of ImageMagick's 'identify' utility for both images:
DSC00021fixed.jpg 640x480 DirectClass 81kb JPEG 1s
DSC00021.JPG[1] 640x480 DirectClass 88kb JPEG 2s

Here's the output of 'file':
DSC00021fixed.jpg: JPEG image data, JFIF standard
DSC00021.JPG:      JPEG image data