Today, I offer a link to "The Meaning of Hakuin's Fuji Daimyō Gyōretsu Painting", by Yoshizawa Katsuhiro.

Paintings of Mount Fuji are common in Japanese art, but Hakuin's painting Fuji daimyō gyōretsu 富士大名行列 (A daimyo procession under Mount Fuji) is unusual in its multidimensional manifestation of the master's thought, achieved through his use of a variety of artistic techniques. It is no exaggeration to say that this piece is the most comprehensive pictorial expression of Hakuin's views on Zen, and is thus the most representative example of his Zen art.

You can see the full painting here.

In The Religious Art of Zen Master Hakuin (Yoshizawa 2009, trans. Norman Waddell), the translation of the classical Chinese poem is a bit different:

Original:
写得老胡真面目
杳寄自性堂上人
不信旧臘端午時
鞭起芻羊問木人

"The Meaning of..." translation:
I have portrayed the True Face of the Old Barbarian
And present it to the priest of Jisho-ji, so far away
If you don’t understand this painting for the festival of December, May 5th
Flog a straw sheep and interrogate a wooden man.

"The Religious Art of..." version:
Having successfully captured the old Persian's true face,
I can now send it along to the priest at far-off Jishō-ji;
If you have doubts about a December Boy's Festival,
Whip the straw sheep forward and ask the wooden man.

The "old barbarian" vs "old Persian" difference has to do with the interpretation of the character 胡. The general meaning is indeed "barbarian," but since it's used here to describe Bodhidharma, who is specifically identified as a Persian in some traditions, it seems fair to narrow it down. See also suiko 酔胡, "Drunken Persians",

What interest me is "the festival of December, May 5th" vs "a December Boy's Festival" for 旧臘端午. Here's how you get the first translation: 旧臘 means "(last year's) twelfth/final-month-in-the-lunar-calendar," and 端午 is the original name for what is today generally called "children's day" (kodomo no hi), a festival held on the fifth day of the fifth month (originally the fifth lunar month, now just May). So "the festival of December, May 5th" is a kind of hyperlocalization of "the festival-held-on-the-fifth-day-of-the-fifth-lunar-month of the final-lunar-month-of-last-year." Waddell's version is a bit better, but that "December" is still quite misleading. Not that there's an elegant solution—English just doesn't have handy names for lunar months. Maybe "a winter boy's festival" would be an acceptable fudge.

What's that? You want to hear more of Yoshizawa's thoughts on Hakuin? Good news! His "Towards a Hakuin Studies" is online too.