| With a function as small as this I doubt it would matter much if you use MySQL or a flat text file. As mentioned, you could measure the difference with microtime, but I can't imagine the difference would be noticable to any visitors.
If you already have a MySQL DB for some functions on this site, adding a new table to it is easy and probably worthwhile. If not, I would just use the text file. If you put all the captions on a new line you can easily load each one into an array using the handy file() function.
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