A great deal has been made by some commentators on Bosch about his "self-portrait". Indeed, some even go so far as to identify the tree man figure on the Hell panel of the Garden of Earthly Delights as Bosch himself. I do not see such a likeness between these two images. However, it did lead me to look a little more into this 'self-portrait'.
It turns out that this drawing in charcoal and red chalk, is one of the folios (fol.275) in a manuscript Recueil d'Arras, Ms 166 in the Bibliotheque Municipale d'Arras in Northern France. This is dated to around 1550 and thus not by Bosch himself. The manuscript has been attributed to a Jacques Le Boucq (ca.1520-1573) and contains many portrait drawings of historical characters - John of Luxembourg (1296-1346), Henry VII (1457-1509), Marguerite de France (1310-82) - whom Jacques Le Boucq could never have seen. Consequently, this image of Bosch is at best a reconstructed one and probably not an accurate likeness.
There is a good article on The Authorship of the Recueil d'Arras by Lorne Campbell in Journal of the Warburg and Courtauld Institutes Vol. 40, (1977), pp. 301-313.
A century later, the French engraver of portraits, Esme de Boulonois, appears to have reworked this image into two engravings.
There is another supposed portrait of Bosch which appears to have been created in the Southern Netherlands sometime in 1585, which is now in Amherst College in Massachusetts. This may in fact be a 17th century painting. The forms on the right side of the face (as we view it) are similar to those in the Esme de Boulonois engraving.