Cloud is on the cover of FFVII
Squall, Seifer and Rinoa for FFVIII
FFIX party for FFIX
Tidus and Yuna in X
YRP in X-2
Full XII party on FFXII...I think, I forget for certain

See a a pttern? It's almost like putting a character on the cover of a game is advertisement... like "you should get this game to be this character!" or something. Crazy, I know but just a hunch.