Let's take a quick moment and define your literary terms before analyzing Hardy's poem:
Imagery: the use of figurative language to appeal to the senses (sight, touch, smell, taste, sound, emotion)
Symbolism: the use of concrete object to develop a second meaning that is figurative to accompany the literal original meaning (ex: an eraser is an eraser, but it could also be used to symbolize a new beginning or destruction)
"That night your great guns, unawares, / Shook all our coffins as we lay, / And broke the chancel window-squares, / We thought it was the Judgment-day / And sat upright"
Imagery: The resonance of the battery guns is so extreme that it breaks window panes and wakes the dead. The incredible noise and vibrations are thus described not through sensory words but through this event, which much more effectively explains how incredible these guns must be.
"It will be warmer when / I blow the trumpet"
Symbolism: This use of symbolism is also allusion, or a reference to another written work. Blowing a trumpet is, literally, just that; however, symbolically, the Bible believes that when it happens, it indicates the apocalypse.
"All nations striving strong to make / Red war yet redder"
Imagery: God does not say that countries are developing their weapons to make war deadlier - instead, color imagery is used to simply create the visual of blood.
"And many a skeleton shook his head [...] Again the guns disturbed the hour"
Symbolism: In this poem, the skeletons are used to symbolize the since deceased, whose death has given them objectivity to find human conflict ridiculous and pointless. The guns, on the other hand, represent the living nations that continuously are in conflict with one another, disturbing the world.