abegail.gielen wrote: ... this is a very important matter to me, because my parents are moving out of the country soon and I would like to keep in touch through Skype...
You have a software problem that will require troubleshooting. With your parents moving soon, I understand your concern.
Because you have been unable to correct the problem using the suggestions that are already posted in iSight discussions, I suggest you make an external backup and then take your Mac to the Apple Store for service.
If you like, the Apple Store may even be able to suggest training that will allow you to troubleshoot and fix future occurrences on your own.