when dealing with images for the web is to carry out any changes in your image-editing program. If you're using PhotoShop do everything there, and then put the images into your site.
The physical sizing of images is easy - work in pixels by the way, not inches. What's important is the resolution, and the file sizes, and you should spend time practising.
Remember that you can reduce the physical size of a large image with no loss of quality, but you can't increase the size of a small image without loss. When you manipulate images digitally you are simply working with fixed amounts of data - an image that contains X amount of detail when small will still contain the same amount when enlarged, you're merely spreading the file information over a bigger area, and you'll lose quality.