How to secure this

Hi everyone There may be blinding bits of total ignorance in this so don't ignore the obvious. This is a security question, but a sentence of background: I'm writing software for a mapping/location website and I want to be able to provide something others can plug into their website that would display their map. So I'm providing a URL like http://www.mydomain.com?h=300&w=250&username=name&password=password The idea is they can define their own height and width and it plugs in as an iframe. That takes the username and password and throws it over web services to get back the data from which we can create the map. My question (and it might be the wrong question) is how can I not give away the password to all and sundry yet still provide a self-contained URL? Thanks in advance :-) Cheers J