Image wont center on posts!

Im having the same problem as many people do, the images i use in my posts doesnt want to align center…Any idea what i can do and fix it?
the url of my blog is… http://www.xboxlife.gr

If you check out my site, click the button Live and see what i mean in the image inside that page, its a work in progress site thats why there is only one image in there..

Thanks in advance and i hope that ill get an answer because im searching for a couple of days on the web for that problem, but found anything that helps. 🙁