@bobknows From the look of this page, apparently yes. When there are multiple styles entered on the same page, the page's code is still executed in order, so the last style would be the visible one. There is a way to alter precedence though.
@DethDude Again, obviously yes, styling codes are allowed on the boards. But I strongly discourage it. For images, I recommend resizing them if they do not fit the DSi's screen. As I said, just a matter of courtesy.