image not centered correctly

Hi,

I am new to CSS and getting problem with the image in center alignment. When i write a post, the image is correctly centered in the visual mode but alignment is not proper after posting. When i click on HTML link the code looks like;
******************************
<TEXT TEXT>
<p style=”text-align: center;”><img class=”aligncenter size-full wp-image-70″ title=”XXX” src=”SOURCE” alt=”XXX” width=”239″ height=”349″ /></p>

TEXT TEXT TEXT TEXT

<img class=”aligncenter size-full wp-image-71″ title=”XX” src=”SOURCE” alt=”DD” width=”236″ height=”329″ />

TEXT TEXT
************************************
I checked into my CSS file an there is no entry for aligncenter. Is this the problem ? Can somebody provide me this css entry so that post will come like;

TEXT TEXT
IMAGE1 //centered
TEXT TEXT
IMAGE2 //centered

Pls help me out.