You can get image Height and Width without loading the image.

Getting image height and width from its header is explained in following articles.

http://stackoverflow.com/questions/111345/getting-image-dimensions-without-reading-the-entire-file

http://www.codeproject.com/Articles/35978/Reading-Image-Headers-to-Get-Width-and-Height

For resizing images in fastest way refer this article

http://weblogs.asp.net/bleroy/archive/2010/05/03/the-fastest-way-to-resize-images-from-asp-net-and-it-s-more-supported-ish.aspx

Also refer this thread to get some alternative methods enlisted.

http://forums.asp.net/t/1085119.aspx

Hope it helps.