Images not getting aligned in posts

Hello,
I have installed a new theme on my WordPress but unfortunately it doesn’t support image alignment in posts.

I have tried many CSS style codes to fix the image alignement problem, but none of them had worked.

The only piece of code for styling images in my style.css are these two lines:

.content .main-post .text .txt img{max-width:470px;height:auto;display:block;margin:7px auto}
.content .main-post .text .txt img[width="20"],.content .main-post .text .txt img[width="24"]{display:inline;margin:0}

How can I make the theme support left, right, center and inline alignments?

Any helps would be greatly appreciated because I had wasted a lot of time trying to fix this.