A change log helps you understand which image was active at a particular time. Without it, later performance discussions can rely on a thumbnail that viewers were not actually seeing.
Record the essentials
Save the previous image, new image, change date, and reason for the revision. Note any simultaneous title, description, promotion, or content changes that could affect interpretation. Keep the actual files, not only a verbal description.
Write a testable hypothesis
For example, you may expect a tighter crop to make a small component easier to recognize. This is more precise than expecting the new design to perform better in every way. State what evidence would support or challenge the idea.
Review with context
When looking at later results, account for changing traffic sources and audience exposure. A simple before-and-after difference does not isolate the thumbnail's effect. The change log makes your reasoning traceable even when the available data cannot provide a firm causal conclusion.