Go U Northwestern (1913)
Song of Northwestern University (Es6)
(Unlikely early. Probably later reprint with newly added ukulele accompaniment.)

Li'l Liza Jane (1916)
Boy Scouts with Ukuleles

I Can Hear The Ukukeles Calling Me (1916)
Dreamy girl under the palm trees

When Old Bill Bailey Plays The Ukulele (1916)
Racist and anti-feminist

Yaddie Kaddie Kiddie Kaddie Koo (1916)
Cupids with ukuleles accompany sexual harassment

My Honolulu Ukulele Baby (1916)
Man and woman under palm tree

Macaroni Joe (1917)
Ukulele and harp (of course only on the cover picture)!

It's A Long Way to Dear Old Broadway (1918)
Soldiers in World War I with ukulele

Broken Moon (1921)
Clown with strange ukulele

It Ain't Gonna Rain No Mo'
Wendell Halls top hit (D6) (1923)

Say It With A Ukulele (1923)
The other top hit of 1923 by Art Conrad

Hula Lou (1924)
Girl with ukulele under palm tree

Ukulele Lady (1925)
Elegant brown-skinned woman in front of palm trees

Under The Ukulele Tree (1926)
Hula dancer and young man with ukulele under palm tree

O How She Could Play A Ukulele (1926)
Wild young woman surrounded by men

There Is No One Else But You
A bigband with Banjo-Ukulele (D6) (1927)

In my Aeroplane (That's built for two) (1927)
Airplane with loving couple

Felix the Cat
Felix the Cat with Ukulele (D6) (1928)

That's My Mammy (1928)

I'll Fly to Hawaii (1929)
Hawaiian woman under palm tree at dusk; airplane is approaching

Help Yourself to Happiness
Probably the most erotic ukulele poster ever! (D6) (1931)