My standard way of inserting an image is much easier:
HTML:
<img src="images/guts1.JPG" alt="some image">
CSS:
img
{
margin: auto;
width: 510px;
height: 388px;
border: none;
padding-bottom: 0;
display: block;
}
The margin auto tells the browser that there is supposed to be the same space left and right, and the display block makes the image actually go into the center.
In your code I didn't have to apply the display block, because you had already created a block level container, namely the #img1.
Of course, if you have more images, and they are not supposed to be positioned the same way, you have to target them in the css:
Example:
#sidebar img { margin-right: 10px;
border: 1px solid red;
}
This will position all images in a sidebar 10px from the right edge and give them a red border.