Image Renditions are great, however they are only supported in:
Publishing site collections
BLOB cache enabled farms

Use this App to manually create your site image thumbnails:
Drag and Drop function
Requires HTML5 capable browser

Canvas and FileReader objects must be supported by the browser
Browsers that support Canvas API
Browsers that support FileReader API

Please note IE8 is not supported and IE9 is support without drag and drop

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