It is, presumably, a matter of taste: would you rather see a Rembrandt-like portrait of Arnold Scaasi, ""New York's last custom designer,"" or an Arnold Scaasi dress? To photographer Barbra Walz, a self-confessed fashion groupie, these 50 ""leading"" American designers are celebrities, and therefore as interesting -- or more so -- on their country-house lawns and apartment terraces as in their workshops or showrooms. ""The real Bill Blass"" is juggling an apple; Chester Weinberg is ingratiating in his undershirt; Diane Von Furstenberg broods in the back of her limousine. But why quibble: those who wear the labels on their sleeves will like this collection of classy magazine-feature photos, equipped with gracious, informed profiles by New York Times fashion reporter Bernadine Morris. ""Gernreich's work is both sociology and art,"" right enough, and Halston ""brought back a pure simplicity."" If only she didn't have to confect something complimentary about so many overblown small talents.