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Magentos standard behavior when you update an image without changing the name of the image is to change the image name for you to imagename_1. If you really just wanted to update the image (f.ex better resolution, extra text in image) this fills your server with dead images over time.
Luckily cobby provides a feature to overwrite images with identical names when the picture is updated by you. The feature can be turned on in the Magento backend in the cobby settings under System --> Configuration --> Services --> cobby --> Advanced.
Set overwrite Images to Yes and changes to an image will update the image instead of creating a dublicate. If the old image name was Imagename.jpg and you upload a new image to the product with Imagename.jpg the old image will be overwritten.
Set overwrite Images to No to follow the old behaviour. Magento will then rename new images by adding an underscore followed by a number.
Don't forget to hit Save Config when you make a change to any setting on the configuration page.