I've now implemented all of the stuff I need to do for the Tumblr API; it was pretty simple, and the usage is even simpler. I created a Post class, with one subclass per type of post Tumblr supports. To post the appropriate kind of thing, you just create that instance, fill in the instance variables, and execute it. Here's the way some example usage looks:

username := 'usernameHere'.
password := 'passwordHere'.
emailAddress := 'emailAddressHere'.

"regular post"
post := RegularPost new.
post email: emailAddress.
post password: password.
post tags: (Array with: 'smalltalk' with: 'tumblr').
post title: 'First Tumblr Post'.
post body: 'This should be posted to Tumblr'.
post post.

"edit a post - note that the post object should have the postId filled in now"
post email: emailAddress.
post password: password.
post title: 'First Tumblr Post'.
post body: 'We just edited the post'.
post post

"photo post"
post := PhotoPost new.
post email: emailAddress.
post password: password.
post tags: (Array with: 'smalltalk' with: 'tumblr' with: 'photo').
post caption: 'This is me presenting in Cologne, Germany'.
post clickThroughUrl: 'http://www.cincomsmalltalk.com/blog/blogView'.
post data: 'PathAndFilenameForPhoto.jpg'.
post post.

"quote post"
post := QuotePost new.
post email: emailAddress.
post password: password.
post tags: (Array with: 'smalltalk' with: 'tumblr' with: 'quote').
post quote: 'A fanatic is one who can''t change his mind and won''t change the subject.'.
post source: 'http://www.quotationspage.com/quotes/Sir_Winston_Churchill/'.
post post.

"link post"
post := LinkPost new.
post email: emailAddress.
post password: password.
post tags: (Array with: 'smalltalk' with: 'tumblr' with: 'link').
post url: 'http://www.cincomsmalltalk.com/scripts/CommunityDownloadPage.ssp'.
post name: 'Cincom Smalltalk NC Download'.
post description: 'Here''s the free Cincom Smalltalk Non-Commercial download - grab ObjectStudio 8.1 and/or VisualWorks 7.6'.
post post.


"authenticate - a misnomer, as it answers info on your permissions, does not authenticate you"
post := RegularPost new.
post email: emailAddress.
post password: password.
post authenticate.

"get the JSON read results"
post := RegularPost new.
post read: username