Hey Chad, welcome to the forums :)
Well you havent actually contained it within anything, so its just floating outside of itself… Before you go any further I would stop you and tell you what you are doing there is probably going to cause you more problems. You are using in-line styling, this is not a good idea – try learning to link to an external style sheet and keep it away from your markup.
Your idea is quite simple, there are a few ways, but here are some ideas.
1) Use a table. Now normally I hate tables, but this is tablitured dater so I dont have a problem with it.
2) use a wrapping div for the link/image pop the number in a span and position it after (therefore under) the link. you can then use text-align:center on the span.
Im sure you might have a few questions after that, but really I would stop now and look at CSS a little more before you continue – it well really benifit you :D