The input is not a valid Base-64 string as it contains a non-base 64 character

Question

Check if your image data contains some header information at the beginning:

imageCode = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...

This will cause the above error.

Just remove everything in front of and including the first comma, and you good to go.

imageCode = "iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...

Very possibly it's getting converted to a modified Base64, where the + and / characters are changed to - and _. See http://en.wikipedia.org/wiki/Base64#Implementations_and_history

If that's the case, you need to change it back:

string converted = base64String.Replace('-', '+');
converted = converted.Replace('_', '/');

We can remove unnecessary string input in front of the value.

string convert = hdnImage.Replace("data:image/png;base64,", String.Empty);

byte[] image64 = Convert.FromBase64String(convert);

Remove the unnecessary string through Regex

Regex regex=new Regex(@"^[\w/\:.-]+;base64,");
base64File=regex.Replace(base64File,string.Empty);