Can I close my gaps with a retainer? If not, what would I need and about how long? (Photo)

If you desire movement of teeth, retainers won't work.  Retainers are used to keep teeth from moving again, so you need orthodontic treatment of SOME kind.Invisalign express does a great job for "orthodontics the second time around".  In only a few aligners your teeth can be where they were, and then wear retainers to keep them there.

Unfortunately new retainers wont have the ability to move your teeth. Retainers only hold the teeth in position. However with clear aligner systems like the invisalign you can move the teeth relatively fast.All the best,