Create image from existing blog, throw exception on failure

When putting this into production, the "die" was causing some debugging errors (why aren't all images on the web perfect...)

In playing around with the different ways to catch & understand the failure, I solved a couple problems and got a nice performance boost with this PR:
* Ditches the local server file write & read (a file that wasn't getting removed anyway)
* Trusts GD's imagecreatefromstring to properly identify the image
* Throws an exception if the imagecreatefromstring fails