Image Uploader API

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This tutorial explains basic use of WordPress media uploader.

http://wp.tutsplus.com/tutorials/creative-coding/how-to-integrate-the-wordpress-media-uploader-in-theme-and-plugin-options

To add image sizes use

 <?php add_image_size( $name, $width, $height, $crop ); ?>

Read more on this at Codex

http://codex.wordpress.org/Function_Reference/add_image_size