Name for a small, central unit of human memory

Technically, it would be engram:

A physical alteration thought to occur in living neural tissue in response to stimuli, posited as an explanation for memory.

A better explanation:

Associative retrieval is an automatic reminding process. It occurs when a cue automatically triggers an experience of remembering. Strategic retrieval is a slow deliberate search of memory to generate hints and cures. Strategic retrieval interrogates the automatic retrieval process.

Encoding the experience strengthen the connections between groups of neurons. The resulting transient or enduring changes in our brains are called engrams. The engram is the representation of a memory in the brain. A retrieval cue induces a pattern of activity: if this pattern is similar to a previously encoded pattern you remember the event.

Non-technically, it would be reminiscence:

• an event that brings to mind a similar, former event.
• an event, phenomenon, or experience that reminds one of something else.
• the act or process of recalling the past.
• a mental impression retained and recalled from the past.

Mnemon a theoretical fundamental unit of memory

(a) memory

2 Something remembered from the past: one of my earliest memories is of sitting on his knee

That 'something' corresponds pretty well to the OP's idea of "a small memory … like an object, picture, or sound."

However, I would definitely prefer a more specific term for the kind of "trigger" that evokes a whole episode. Let's see what the other have to say.

Try recollection. This word refers to the things brought back by memory.

Mnemon and engram intend to refer to insignificant, atomic units, not something like a mental picture.