You can't convert a string to a byte array directly, because a string is (normally) made of unicode characters, which don't "map" directly to single bytes - they are variable length values.
You can do it, but you have to tell it explicitly what type of data the string contains:
string s = ...
byte bytes = System.Text.Encoding.Unicode.GetBytes(s);
But...unless you have been a bit silly, your image should not be in a string based column in your database at all: it should be in a byte based datatype instead.
See here: Why do I get a "Parameter is not valid." exception when I read an image from my database?
] - it talks about how to insert and extract images from your database correctly.