If it isn't obvious, the photo comes from the unveiling party for the F1 LM which McLaren held at the Genesis factory in late 1995.
A few years ago Le Man shared an additional image from the event sourced from Autosport magazine, along with scans of Ron Dennis' speech to the crowd that evening. I checked the original thread they were featured in and two out of the three image links were dead now, so I've rehosted them below for the benefit of the LM archive.
>8^)
ER