You cannot set the imgsrc or avatar of an object unless you use an image that is in your Roll20 Library.

This is probably a simple fix, but I cannot seem to find an imgsrc that will satisfy the API.  I am using the getCleanImgsrc utility function (found in the documentation) along with a function that posts the imgsrc of the selected token to chat. Even with that, the error persists. The images are obviously in my library in these circumstances. Is there something I'm missing? 

Try logging the result of getCleanImgsrc to the API Console Log.  If it's returning a URL, it's usable.  If it isn't there is something wrong with the url.  Paste the URL you're passing in and I'll tell you what it is. Also, be certain you're actually getting this error because of a bad URL and not just because you've left the avatar on a character you're creating blank.  This is a slightly frustrating error because the DEFAULT value of '' (empty string) is invalid by the validation step, resulting in this message.  You see this especially when creating rollable table items as they have the opportunity to associate an image, yet you often don't want to and thus the default image is specified, resulting in this "error".

In addition to Aaron's points, is the token's sourceimage stored in your library, or is it marketplace content? TheAPIcan only assign imgsrc's that you uploaded (or are stored in another user's library).

Dustin said: The images are obviously in my library in these circumstances. Good point Scott, "my library" is a bit ambiguous here.