JQuery :: Passing Base 64 Encoded String Through Ajax?

I have an image that is encoded as a base 64 string, and I'm having trouble passing that a WCF service using $.ajax(). I initally tried using JSON to pass the argument to the web service, but I kept getting 400 Bad Request errors for anything of reasonable length (if I just pass a test string in, it makes it through, of course). I've tried calling encodeURIComponent on the string before stringifying it, but that hasn't helped. I've also tried various content types ("application/json; charset=utf-8", as well as whatever the default is) and that hasn't made a difference either. Unfortunately, the request is being made through a mobile phone, so debugging options are few. Is there an upper limit to the size of the argument that I'm passing in this? The strings can be a few hundred kb at the low end up to maybe 1 or 2 mb at the high end.

Here is a snippet of the code I am using:

$.ajax({
type: "POST",
url: "http://www.myserver.com/MyService.svc/MyFunction",
cache: false,

[Code]....

No matter what the string consists of, it errors would with a 400 Bad Request error. No further information.