想象:imagine(as a verb);image(as a noun)
思议:Literal meaning is thoughts and comments about something
But “思议” is rarely used alone in Chinese ,we often put "不可”with it as an idiom “不可思议”.
不可思议:inconceivable、unimaginable
And you can also put"难以”with"想象”,
难以想象 is the same meaning to"不可思议”。