Desgraciadamente CSS PIE no soporta actualmente background-size:
Cita: While the PIE parser will allow them, the following aspects of the background shorthand will currently be ignored when rendering in IE 6-8:
background-attachment (will always use 'scroll' even if 'fixed' or 'local' are specified)
background-size (will always use the image's intrinsic size)
background-repeat values of 'space' or 'round' (the other repeat values are supported)
background-origin (will always use 'padding-box')
background-position values with more than 2 parts
Support for these items will be added in future versions as possible.
Ver en:
http://css3pie.com/documentation/sup...pie-background
Acá
http://nooshu.com/jquery-plug-in-sca...ckground-image hay un polyfill en jQuery para background-size.
Lo que podrías hacer es detectar si background-size está disponible mediante Modernizr y aplicar el plugin jQuery solo sino está soportado.