Is there a way to auto-resize images based on one side?

On my blog, College Info Geek, my content area is 490px wide. Every time I upload an image that needs to be larger than this area (at least in its lightbox), I have to open the image’s options box, use a ratio calculator to figure out the right Y-dimension for a 490-px width, and manually change these dimensions.

Is there a way I can have WordPress do this automatically?

Here’s an example of the code my theme generates for an image (the link opens the image in a lightbox, full size):

<p style="text-align: center;"><a href="https://collegeinfogeek.com/wp-content/uploads/2011/11/6.png"><img class="aligncenter size-full wp-image-3962" title="Slacker Radio" src="https://collegeinfogeek.com/wp-content/uploads/2011/11/6.png" alt="Slacker Radio" width="490" height="379" /></a></p>