The original edition of the first novel, Pantagruel, was most likely published in 1532 by Claude Nourry in Lyon and only one copy of this edition has survived. This copy is in the archives of the Bibliothèque nationale de France (BNF), where it is identified as RES-Y2- 2146 (Pantagruel, ed. Defaux: 19). A scanned version of this 1532 edition can be browsed online. It has an illustration on the title page and decorated initials at the start of each chapter. Some words have been crossed out by one of its earlier owners, possibly as a form of censorship (Pantagruel, ed. Defaux: 19); see for example this page. This edition does not have any other type of illustrations. (This edition also has two chapters IX. (Pantagruel, ed. Defaux: 13).) Pantagruel was very successful; it was reprinted several times between 1532 and 1542 (Pantagruel, ed. Defaux: 15).
A scanned version of a 1533 edition of Pantagruel can also be found on the BNF site. This edition also has decorated initials at the beginning of each chapter but no other illustrations.
The second novel, Gargantua was first published in 1534. A scanned version of the 1534 edition of Gargantua is also available on the BNF website. This edition has an illustration on the title page and decorated initials, but no other illustrations.
The BNF also has a scan of another edition, dated 1534 in the search results, with narrower (or higher) pages. This edition does not have an illustrated title page (to me, it looks like the original title page was lost and replaced with one that was printed later; it just says "Gargantua").
There is also a 1535 edition of Gargantua, which has a decorated title page and decorated initials, but no other illustrations.
The BNF also has a scan of a 1542 edition of Gargantua that even mentions the year of publication on its title page. This edition has an illustration on the first page of the "prologue", on the first page of chapter III and on the first pages of some later chapters (for example this one).
The BNF also owns a set of pages with illustrations dating from 1537.
The third book, known today simply as Le Tiers livre, was published after a long break, namely in 1546, printed by Chrestien Wechel in Paris. A scanned version of the 1546 edition of Le Tiers livre (with the year of publication on the title page) is also available on the BNF website.
Its title page does not have an illustration; it uses decorated initials only on the first page of the "prologue" and the first page of the actual narrative. (I assume this is intentional; otherwise, it would give the impression that the creation of decorated initials had been abandoned early in the printing process.) There are no illustrations.
The BNF's search results also contain a link to a version available at the Bibliothèques Virtuelles Humanistes. (This is another copy of the edition printed by Chrestien Wechel in Paris. Lazard points out that Le Tiers livre was printed several times in 1546; see Lazard: 84.)
A 1547 printing of Le Tiers livre by Claude La Ville is also available at the BNF. This edition does not have an illustrated title page but it has illustrations on the first page of the "prologue", the first page of the first chapter, the start of the second chapter and each of the other chapters.
The BNF has an edition of Le Quart livre printed in Lyon in 1548 (the date is on the title page).
This edition has an illustration at the beginning of the prologue, the beginning of the first chapter, the beginning of the second chapter, the beginning of the third chapter, and the beginning of later chapters. (Decorated initials are not used consistently. For example, chapter VII begins with a simple drop cap. Where decorated initials are used, they vary in size: compare, for example chapter VIII and chapter IX.)
The 1548 edition of Le Quart livre was actually an unfinished work (Lazard, 138). The 1552 edition of Le Quart livre was published by Michel Fezandat in Paris. It has an illustration on the title page, decorated initials, but no other illustrations.
The last novel, Le Cinquième livre was published posthumously in 1564. Some scholars have questioned its authenticity but it is reasonable to assume that it was based on an unfinished draft by Rabelais. Archive.org has a scan incorrectly dated 1550. It has decorated initials but no illustrations.
The BNF also has a 1564 edition of Le Cinquième livre (dated 1564 on the title page). This edition uses decorated initials at the beginning of the "prologue"; the chapters begin with ordinary drop caps. The book contains no illustrations.
The set of early illustrations that is probably best known to modern readers is that in Les Songes drolatiques de Pantagruel, which Richard Breton published in 1565 and attributed to Rabelais. Present-day scholars don't believe Rabelais created these illustrations.
Conclusion: The very first editions of Pantagruel, Gargantua, Le Tiers Livre and Le Cinquième livre were not illustrated (except for decorated initials and/or an illustration on the title page). However, later editions sometimes added illustrations.
If the 1548 edition of Le Quart livre at the BNF is indeed a copy of the very first edition of that novel, that book had illustrations when published for the first time.
Sources (besides BNF):
- Lazard, Madeleine: Rabelais. Pluriel, 2012.
- Rabelais: Pantagruel. Edition critique sur le texte de l'édition publiée en 1534 à Lyon par François Juste. Introduction, variantes et notes par Gérard Defaux. Livre de poche, 1994.