The "Prologue" of Geoffrey Chaucer's The Canterbury Tales gives readers a distinct picture of the characters (or pilgrims) on their way to visit the shrine of Thomas Becket. Challenged by the owner of the inn, the pilgrims must tell a tale of morality. Ironically, each tale told highlights the teller's own vices and fallacies.

As for The Canterbury Tales acting as a picture gallery, the "Prologue" introduces each of the pilgrims describing them using both physical and internal traits.

For example, the "picture" of the Monk details a man with big, sparkly eyeballs. He is both bald and fat. Unlike what would be expected of a monk, the Monk wears fine fur-trimmed clothing and expensive riding boots. His face is both tanned and greasy. 

His head was bald and shone like any glass
And smooth as one anointed was his face.(35)
Fat was this lord, he stood in goodly case.
His bulging eyes he rolled about, and hot
They gleamed, and red, like fire beneath a pot.

Each character in Chaucer's tale is depicted in the same fashion. While some are described using more detail, all of the characters are described using such detail that one could create a mental picture book of each.

How is Chaucer's The Canterbury Tales a portrait gallery?

Although we have visual depictions of most of Chaucer's pilgrims in various illustrated manuscripts of The Canterbury Tales, Chaucer includes very detailed "portraits" of the pilgrims in the Prologue so that  readers have no problem visualizing the speaker of each tale.  Because each pilgrim is meant to be both a specific individual and a representative of a class of people, Chaucer includes in his narration details about each pilgrim's occupation, appearance, character, and place of residence so that reader's can picture them in the mind's eye, as well as understand their character through description and the tale each one chooses to tell.

For example, the first pilgrim to speak--because he occupies the top level of society within the group--is the knight, who his described as 

A Knyght ther was, and that a worthy man,/That fro the tyme that he first bigan,/To ridan out, he loved chivalrie,/Truth and honour, fredom and curteisie. (A.ll.43-46

Chaucer provides the reader with the character of an ideal medieval knight for whom "Truth and honour" are important attributes of his inner being.  Equally important is the physical description of the knight:

His hors were goode, but he was nat gay./Of fustian he wered a gypoun/Al bismotered with his habergeoun,/For he was late ycome from his viage. . . .

Because the knight has just returned from one fo the Crusades, Chaucer makes sure the reader "sees" that the knight horse is a fine one but not adorned in any way and that the knight himself is dressed in a plain tunic, with his chain mail completely covered with dirt.  This physical detail is important not only because it allows the reader to visualize the knight but also because the knight's physical characteristics mirror his inner character--he has, because he believes in "Truth and honour" been to fight on Christianity's behalf.

Another character--the wife of Bath--is depicted as follows:

Boold was hir face, and fair, and reed of hewe./She was a worthy womman all hir lyve: Housbandes at chirche she hadde fyve, . . . Gat-toothed was she, soothly for th seye.

Here, we have a physical description--a reddish complexion and a "bold" look--as well as a description of her life, specifically, that she had five husbands.  This, of course, implies a lusty, healthy woman who can outlive at least four men.  That she is a lusty woman we learn from Chaucer's detail that her front teeth had a gap between them.  In medieval beliefs, a gap-toothed person characterized as vain, lustful, bold and given to having sex as often as possible.  In very few words, then, Chaucer gives the reader both a physical description and a glimpse into the wife's character.

Chaucer carries this style through for each of the other pilgrims in such a way that allows us to visualize each one and to understand the essence of each pilgrim's character.