I believe the purpose is to provide a part of the url that will change when the image is updated. That way browsers can cache the image but still know when to reload it. Normally a browser will cache an image but send a request to check if the file has changed or not, but since those thumbnails are saved in the database it would be a lot of work to run a php script just to check the date when it last changed. That would kind of defeat the purpose.
So I guess the short answer is that you probably shouldn't remove it.
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