Flickr upload by email to Tumblr

You can't.

It's unfortunate, because there used to be a workaround using Picasa (details below), but it no longer works.

This is all moot anyway with the move away from Google+ Photos to Google Photos. Google Photos doesn't have an email option either, but there are other ways to automatically get photos in there.

The steps below no longer work, but are left as a legacy.

It used to be possible. The key is that you don't do it through Google+, but rather Picasaweb (soon to be Google Photos)

1. Go to Picasaweb
2. Click the gear in the upper-right corner and select "Photos settings"
3. On the "General" tab, there is an option for "Upload photos by email:"
4. Click the checkbox and select a "secret word". The combination of the secret word and your user ID will create a special e-mail address (i.e., {userID}.{secret}@picasaweb.com)
5. Use that e-mail address to e-mail images to your Picasaweb (and, thus, Google+) photos

The photos appear in the "Drop Box" album, which is the default for newly uploaded photos, and the e-mail subject is used as the caption.

From here, you'll want to move the image to an album you're already sharing or, if you want to share every image you send via e-mail, you can share the Drop Box album with whatever Circles you want and it'll appear in your Stream.

Note: Google recently changed picasaweb.google.com to redirect to your Google+ Photos. While they do provide a link to get back to Picasaweb without redirecting there's no telling how long they'll keep that link around. There is not, at this time, a way to upload photos to Google+ via email.

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