Yes, you can use the height and width attributes, e.g. <img src=logo.gif alt=ACME width=50 height=20>. You can also set the height and width properties in CSS.
When the width or height differs from the dimensions of the image itself, browsers rescale the image.
However, it is almost always better to scale the image in your favorite image processing program. Scaling down a large image that way lets you control the quality of the result, and it also saves bandwidth (and processing time in browsers).