Screenshots are visible on Day view. By default, they will be visible in the tooltip.
If you press F12, they will show up in bottom half of ManicTime.
Moving your mouse over the timelines will update the screenshot.
By default, ManicTime will take a screenshot every 30 seconds of any application you use. You can restrict ManicTime to only take screenshots of specific applications. For example screenshots are very useful in remote desktop applications like Remote desktop, TeamViewer, where ManicTime doesn't show a lot of data.