I note this on this page: http://support.webvanta.com/support_article/615733-creating-and-accessing-image-renditions (and i believe it is out of date and new instructions are needed)
The renditions are generated when you upload an image, unless you uncheck the box in the Processing Options section of the upload dialog labeled "Generate Thumbnails".
If you want to regenerate the renditions for an image you have already uploaded (if you have changed the Config Setting that determines what renditions are generated, or didn't check "Generate Thumbnails" when you first uploaded the image), select the file in the asset list (Content > Images and Files), click Update File, make sure Create Thumbnails is checked in the Processing Options section of the dialog, and click Save. Note that you do not need to specify a file to upload, if you want to work with the already-uploaded file as the master.
EXCEPT, i am unable to select the file and then generate renditions for all inclosed images that originally had the button unselected. is there an easy way to do this now rather than going to each individual image and re-checking the "create renditions" checkbox? Or am i missing something?