If you research older threads and the "current residents" section on the websites of U.S. programs you can get a pretty good idea about the background that IMG's who matched into ophthalmology have. I know a handful of IMG's in ophtho and other competitive residencies - while there is no general rule as to what "gets you in", it is probably fair to say that you will need to look at the stats of successful U.S. applicants and prepare yourself that as an IMG you will need higher scores, more convincing LOR's and/or more impressive research to match. You will also need to be very flexible in terms of location, prestige of program and spending some extra years doing a pre-residency fellowship or residency in another specialty.
The fact of the matter is that depending on the country where you completed your medical school (not wanting to be judgemental, but looking at it realistically not all IMGs are viewed the same) there may be more or less degree of uncertainty regarding the equivalence of quality of medical education. Programs have plenty of qualified U.S. applicants to choose from whose medical schools they are familiar with, so if you come from an educational system/med school that doesn't mean anything to the person who reviews applications, you will need something that stands out in your application to make it past the screening and very good interview skills to get ranked.
I hope this is somewhat helpful and doesn't sound too discouraging, but I think it's better to be mentally prepared for the challenge than to get disappointed and frustrated in the process. Guess the bottom line is it's very difficult but not impossible.