I think the biggest factor that is used to characterise suffering in this poem is that of indifference. The various pictures that the speaker looks at as he wanders up and down the gallery characterise suffering as something that occurs alongside other humans that really just don't care about the suffering that is being experienced. Consider the following lines:

About suffering they were never wrong,
The old Masters: how well they understood
Its human position: how it takes place
While someone else is eating or opening a window or just walking dully along;

The "old Masters" therefore new the reality of suffering. One man's suffering is something that others just ignore. The world does not stop for our suffering, and, however great and intense that suffering is, people remain indifferent to the trials of one person and the world goes on. An image therefore would need to contain symbols relating to this central fact: perhaps you could use some of the ideas from the poem itself, which is, let us not forget a poem that describes various paintings and the indifference that the suffering characters in these paintings face from their fellow humans and other animals.