preview and iphoto are both useless for it no text tool. but comic life could work if you got that with your mac. also if you search download.com there should be some caption apps there for mac
I also guess you dont have photo shop?
or iwork or word? how ever a work around is to drag a image into textedit then write your cap under the image then take a screen shot of that window then open screen shoot in preview then crop out every thing but text and picture
 done hard way with textedit and croped in preview
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