Table centered in dreamweaver, not in browser

Question

I have the following code and css.  In Dreamweaver, in design view, the table is centered, as expected, but in Safari and Firefox (the only two I've tried), the table is left aligned.  I don't understand.  Thanks for help
<table align = "center">
<tr><td id="firsttile"></td> <td id="secondtile"></td>
</tr>
<tr><td id="thirdtile"></td> <td id="fourthtile"></td>
</tr> </table>
with css for the 4 tiles are the same, here is for firsttile:
#firsttile {
transition: background 1.0s linear;
background-image: url("adultstrab.jpg");
background-repeat: no-repeat;
background-size: 100% auto; /*or your image's width*/
height: auto; /*or your image's height*/
margin: 0;
padding: 0;

Answer 1

Bascially what I have is a class .thumbnail and i put want to put 6 jpgs at the bottom of my screen each with the code following the css below.  I'd like the jpgs to be centered, no matter what size screen.  I can center them with css for a particular screen size, but I'd like a way for them to automatically center no matter what size screen.  Any way to do this?
.thumbnail {
position: relative;
bottom:0px;
float: left;
border: 5px solid red;
margin: 1%;
padding: 1%;
width: 4%;
margin-bottom: 2px;
margin-top: 2px;
   <div class="thumbnail"><a href="amb.jpg" target="_blank">
<img src="amb.jpg" width="100%" height = "100%"></a><br>
</div>
Then 5 more of the same, with different jpgs.