Close the duplicate we've been working with and make a new duplicate off the original. Choose the Image Size command. Set the resolution to 72, constrain proportions, resample checked, and Bicubic Sharper as the resample method. Since we need the image no more than 600 pixels high, go ahead and enter 450 in the pixel dimensions height (we'll go smaller since most browsers have title bars, menus and scroll bars that also take up screen space). Notice that the file size decreases considerably.
Click OK and set the magnification to 100% to see the image at the actual size it would appear on the Web. I mentioned before that resampling results in a loss of quality. This is still true when you are resampling to decrease the pixel dimensions, as we have done here, but if you look at the detail areas in the image, the loss of quality is not nearly as noticeable. In general, resampling to a smaller size introduces some softness which can usually be corrected by using the Bicubic Sharper resampling method, or by following the resampling with a bit of sharpening.
