"Hi, I'm from (Enter organization/site/group/etc here). We need (Enter Type of work needed here) for our (Enter Organization/Site/etc here). We don't have any way to pay you for this, but you'll get free exposure from our (Enter Organization/Site/etc here)'s (Enter Website/Game/Service/Etc here)."
In other words the standard "We need work, we don't know any artists, we can't afford to pay for custom work, we'll trade you your hard work for the "exposure" you'd get for putting it on their site."
I've been getting this in SL lately. People want to use my avs in magazines, and ballets, and machinima, and other stuff. Do they want to pay for it? Hell no. They want me to give it to them for the "exposure".
Let's try this logic in a real world scenario, shall we?
"Hi, UPS? I need some packages delivered, but I can't pay for them. But, just consider this for a moment. If you deliver /my/ packages /for free/, you'll get exposure from people seeing your vans running around town!"
You know what UPS would tell you?
"Fuck you." (Likely in much politer terms).
Consider this when an artist tells you the same, in similiar terms.