> The iPhone is the best selling model in the United States.
That's an interesting (and slightly misleading) way to present that statistic. Apple has the best selling models, but does not have the majority of the market by operating system, which is what matters here. It's close, but Android is still supposed to have over 51% of the US market.[1] (If the graph itself is occluded, see the summary below it).
> It is on College Board to support its default image format.
It's a minority platform. It's only a slight minority, but it is one. If you want to make a case that they should support any default format for a platform over a certain percentage of usage (or "almost half"), that's fine, but you can't rely on the obvious argument that it's the dominant platform and thus should be supported, because it isn't the dominant platform.
Edit: Whoops, forgot to include citation
1: https://www.statista.com/statistics/266572/market-share-held...