When the image is big, this causes allocation of a lot of memory (e.g. for 30MB image, the allocated memory for the string is ~300 MB), and when the base memory consumption of the process is high, this can cause OutOfMemoryException. 

Instead, the conversion to base64 format could be done using Convert.ToBase64String(Byte[], Int32, Int32) on smaller chunks, which are immediately written to the text stream and left to be garbage collected.