Do not resize images in browsers; this degrades image viewing quality. Resize your images in an image editing program such as Photoshop, then specify each image size at its actual pixel dimensions (width and height) for the browser.
Each image should be presented at 100% of its original proportions. Do not stretch or squish images to fit a particular area of your web pages. If you need an image at different proportions than your original image, either crop the image to fit the space at 100% proportions, or select an alternative image that better fits your web page.
In addition to using 100% dimensions and proportions for your images, please make sure that your images are prepared at 72 dpi in your image editing program. Browsers have to "interpret" images at other sizes, and the results are undesirable.
Finally, larger images are not necessarily better – in fact, they often distract from the professionalism and usability of your site. Remember that people use browsers of different sizes, so a 4x6 image on your browser may take up most of someone else's screen.