With pretty much all browsers and web site editors, setting the image size in html (in most cases this is what the size controls do when you add an image) doesn't actually alter the actual image file, it simply shrinks or stretches it on screen when it is displayed. This is a BAD thing!

If your image is too small, stretching it to make it bigger just creates image artifacts - which make the image look bad and make your site look amateurish.

If you image is too big then shrinking it to make it look smaller has 2 negative effects - firstly the algorithm used by browser to shrink an image is pretty dumb, so there will be a visible loss of quality, even though your original image file is is good. Secondly you are making visitors to the site download the big image file, even though they are getting just a small version of it to look at - so it is using up lots of extra bandwidth and slowing down their browsing experience. In many cases we see, people put original digital photos on a site that are a few Mb in size, but shrink them in the page to a small image  that would normally take just a few tens of Kb. Instead of loading in a second or two such pages can take 30 seconds or more to load. If you see an image being drawn slowly when you have a fast internet connection it's because of this... you shoud contact the webmaster and complain!

The right thing to do is ALWAYS size an image correctly for the page using an image editing tool such as Paint Shop Pro, Picassa, Microsoft  Picture Manager or Photoshop, then load it into the site. If you want several versions of the image at different sizes then create several versions with different sizes, stating their size in the file name.

There is a Microsoft Powertoy for Windows XP that does bulk resizing here.

 Another powerful tool is ImageMagick - an opensource, command line tool that offers a range of bulk image editing capabilities. Download it or read about it.