Insert into varbinary(max) failing

Hi,

I am trying to insert an image and 2 thumbnails into varbinary(max) fields via a stored procedure and I am getting an error returned. If I execute the the SP direct and provide nulls for the fields then the row is created so I suspect the SP and Table are Ok. I also list the byte array sizes when it fails so I can see the buffers have some data, eg.

buf1: 61896
buf2: 11611
buf3: 5174

The c# code is:

SqlCommand objCommand = new SqlCommand();
// Add user id and display order (ints)
objCommand.Parameters.Add("@picTenId", SqlDbType.Int).Value = strId;
objCommand.Parameters.Add("@picOrderId", SqlDbType.Int).Value = "1";

// create and add byte array buffers
byte[] bufM = new byte[0];
bufM = new FixedResizeConstraint(620, 410).SaveProcessedImageToByteArray(strImageF);
objCommand.Parameters.AddWithValue("@picMain", bufM);
byte[] bufT1 = new byte[0];
bufT1 = new FixedResizeConstraint(200, 132).SaveProcessedImageToByteArray(strImageF);
objCommand.Parameters.AddWithValue("@picThumb1", bufT1);
byte[] bufT2 = new byte[0];
bufT2 = new FixedResizeConstraint(116, 77).SaveProcessedImageToByteArray(strImageF);
objCommand.Parameters.AddWithValue("@picThumb2", bufT2);

// run the stored procedure
Int32 lngResult = utlData.Run_Procedure("hex_setPicture", objCommand);

The function adds a final 'result' output parameter to the command object, hopefully to return a new row Id. The function works well with inserts to other tables so I also don't suspect a big problem here.

The function returns -1 which is my error setting.

I'm a bit stumped as I've tried every thing I can think of - any ideas would be gratefully received.