Every time i used to upload files in the width size of 500 according to wordpress theme, but it made me irritating and RSS readers cannot view the picture in full size. So i found a way to display pictures in a fixed wdith for wordpress theme via CSS

Find out style.css in your theme folder and search for content selector. For instance search for img in your style.css file and there will be content selector.

Example

.entry img
{
margin:0 10px 10px;
}

Append the following CSS code with the above code

max-width: 500px;
width:expression(this.width > 500 ? 500: true);

Finally the code will be

.entry img {
margin:0 10px 10px;
max-width: 500px;
width:expression(this.width > 500 ? 500: true);
}