CRM SDK Annotations for Activities only returning part of image

Hi,

I'm using the following to retrieve an annotation document body and save to a file:

byte[] fileContent = Convert.FromBase64String(retrievedAnnotation.DocumentBody);
fileStream.Write(fileContent, 0, fileContent.Length);

The issue is that the document body doesn't seem to hold the full image. When I try to save something like a png I am only getting the top 8 or so rows of pixels and the length of the byte array is always 1500, regardless of the size of the image that is held in the annotation. Is the 1500 a default that I cannot see and needs to be changed? Is there a better way of getting the body? 

Usually I can google my way out of these things but I'm kind of at the end of what I can find.

Thanks.