Return key behavior

I've got an ASP.NET gridview that I've modified by causing an "insert" row to appear in the middle of it.  The insert row has one text input field (and variations may have several).   This is all done in javascript.    I don't think the ASP.NET aspect matters but I thought I would describe it to be clear.

What I now need to know is how I might control the return key behavior in this situation.  I'd like the return key to do nothing if the focus is on the text fields.  Then, when the focus is on an <img> that I'm using as an "enter" key, I'd like the enter key to essentially "press the button" (or fire the associated function).

Any guidance on this would be appreciated.

Thanks!