I was having the same problem as well, then somehow one of my images showed up as the thumbnail without me doing anything There was no set album thumbnail for me.

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1985 Chevrolet Monte Carlo SS
5.7 LITRE / TES Headers / SShaker Hood / WC T-5 5-Speed
1970 Pontiac GTO
Orbit Orange Judge Re-creation / YZ RAIII 400 / M21 4-Speed