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