I came across this image on Simon Lamb’s Gazzetta Della Bici blog, who in turn found it on a facebook group called 80s cycling remembered. The group has over 3000 images of 80s cycling. This one was added by Duff Fawcett but there is no indication who the photographer was or what the source is.
Wikipedia tells us that in 1988, Millar rode for the French Fagor team and managed his best position in a one-day Monument Classic, third in Liège-Bastogne-Liège, which is the race pictured above.
In the Tour de France, he lost the opportunity of another mountain stage win in Guzet-Neige when, sprinting uphill to the finish with Phillipe Bouvatier, both riders mistook a gendarme’s signals, took a wrong turn and ceded the win to Massimo Ghirotto.
From what I remember of Richard Moore’s book, 1988 was an otherwise uninspiring season.