OK, it can make some sense. Here is what you can do:
First of all, images can have different aspect ratio. So, first of all, decide if you want same width or same height. Even if the aspect ratio is the same, always specify only width or only height, never both — just in case. Or, the width of one image should match the height of another one, or any other relationships between sized. It should be defined by your layout and design.
Specify it in the image tag (not table elements or something like that):
<img src="myFile.png" width="400" alt="My image" />
or
<img src="myFile.png" width="500" alt="My image" />
Try to make it so the original image size is only the same as the rendered size or a bit bigger, so it can only scaled (re-sampled) down, not up. Your image will be re-sampled automatically by the browser. Well, minor enlargement can be satisfactory, but any considerable enlargement can look really ugly, even if you use the finest off-line algorithm.
In all cases, it's the best to keep only minor level of re-sampling in browser. The ideal situation is the exact size match with the source image. If you scale down too much, you waste considerable amount of bandwidth. If you scale up just a bit to much, it's even worse — please see above. If you need to change rendered image sizes, try to replace the source images with the images of different size.
—SA