i am a licensed, board-certified ophth in the US, but once i too wanted to study and work in spain. i even have a valid medical license to practice there.
first, you should do a med school rotation to find out if indeed you want to do residency, work there, etc. It is probably a better idea to do a residency in the US and then a fellowship there. easier to get, less formalized, you could return to the US (if need be) and your fellowship training would be recognized just like a US fellowship (that is to say neither ACGME or ABO accredited)
As for centers:
Vissum in Alicante with Dr. Jorge Alio
Oviedo has a large Instituto (fernandez vega, i think)
Barraquer in Barcelona
these are private, the gov't residencies are at all the large hospitals in madrid and barca, like Ramon y Cajal, llobregat, etc. Additionally, i don't think you can work in a gov't hospital in the INSALUD System unless you are an EU citizen (not 100% sure)
You should be aware that residencies in continental europe are not like the US. there is no set time frame nor set surgical volume. it is much more like
an apprenticeship, potentially with vastly less surgery, potentially lasting 5-8 years. even the term ophthalmologist does not mean the same. some are medical, others surgical.
in summary, spain's level of ophthalmology is as good as anywhere, but entering into the system is difficult as a resident. Fellowships offer much more flexibility, but they will likely be unpaid. You should look at Moorfield's in London for additional opportunities. docs from all over europe work there, and the surgical volume is muy grande (again, better as a fellow).