Its because the Encoder is not asynchonous and it can take a while to encode a large image; especially jpegs and especially on mobile.
There are asynchronous encoders out there but self promotion is not allowed. hint, hint.
edit: rules be damned! heres a link to my blog post (and code) on async encoding: http://blog.leeburrows.com/2011/01/saving-an-image-to-hd-with-actionscript-part-2/