The thread I linked explains you need to setup a cron script on the server and shows how. You may need to contact your webhost about more info on how to set a cron task on your specific server.
If they get an generic error that indicates your server is crapping out on something. Now you would need to open a new thread on that issue but no doubt we would need more info. Like are they trying to upload an image thats too big for the server to process etc
You can post a url and a test user login here and we can try posting an ad.