Normally, the problem you get is "Parameter is not valid." which indicates that the data in teh database is not a valid image. Often the problem is actually caused when the image is loaded into the database, particularly if the image is saved by concatenating strings to form an SQL command:
string sql = "INSERT INTO myTable (imageData) VALUES ('" + myImage + "')";
Actually generates an sql string:
INSERT INTO myTable (imageData) VALUES ('System.Drawing.Bitmap')
Which won't throw an error, but also doesn't store the actual image data.
Check your DB, and if this is the case use parametrized queries to load the DB instead of string concatenation (you should be doing that anyway as a matter of course).