Try out this for better understanding.
DECLARE @Img varchar(50)
SET @Img = (SELECT ImageName FROM MyTable WHERE Id=1)
IF @Img IS NULL
BEGIN
PRINT 'Image is NULL'
END
ELSE IF LEN(@Img) < 0
BEGIN
PRINT 'Image is blank'
END
ELSE
BEGIN
PRINT 'Image is available = ' + @Img
END
Hope it helps.