Is it possible to upload an image to my Google+ profile via email?
All other social services seems to have this feature available, but I can't find it on Google+.
Yes. The key is that you don't do it through Google+, but rather Picasaweb (soon to be 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