If you're using the import image plane option, then you can open up the image plane attributes, then scroll down in the attributes till you see the placement section. At the top of that section is a drop down box. Usually it can get distorted if set to "Fill" or "To Size". Check to see if it's on best fit. There are also a couple of buttons below that drop down box that lets you set to resolution gate or film gate. As far as a universal setting, I'm not sure.

Edit: Ok, I just checked the help files, looks like Maya is supposed to maintain aspect ratio for everything except the "To Size" option. I didn't see anything about a global setting though.