Imagery is describing using the five sense of sight, smell, hearing, taste, and touch. If you can see, hear, taste, smell, or touch an item in a poem, then it is an image.
In this poem, Donne's speaker uses startling and violent images to depict love. Love is harsh and painful, according to this speaker. He likens it to the plague, a fatal disease that killed many people. He compares it to a burning flash of gunpowder.
Donne's speaker also compares love to a pike, a pointed staff that is used as weapon. He says that the pike of love spears a heart, which it then fries. We can both see and feel a heart speared and then sizzling over a fire.
When the speaker's heart is broken because his love is not returned, he uses the image of a mirror smashed with a blow that reduces it to shivers (shards) of broken glass.
As you go through the poem, you will be able to find more grim images of love.