It really depends on the type of image how CPU intensive reading your image is. And the more pixels you have the longer it takes to resize.
It does make sense to have threads but maybe you should use the LimitedConcurrencyLevelTaskScheduler (an example can be found here: http://msdn.microsoft.com/en-us/library/ee789351.aspx) to limit the number of threads.
It does make sense to have threads but maybe you should use the LimitedConcurrencyLevelTaskScheduler (an example can be found here: http://msdn.microsoft.com/en-us/library/ee789351.aspx) to limit the number of threads.