There seem to be a glut of image sharing services geared for Twitter or the like, which of them should I stay away from if I care about the picture and which allow me to retain control over the photos?
The only reason that services which allow sharing option (TwitPic, imgur and even Dropbox) force you to share rights on your content is to allow the service to do its job, namely: publish this content.
Dropbox, for instance, had an incident when they changed the TOS: there was a big discussion, and Dropbox guys had to explain why they did that.
Here the citation from their explanation:
So in the copyright world services have to protect from copyright owners to not being sued.
The short answer: if you want to share copyrighted work and retain full control, you should accomplish this with your own server, the only way you set the rules. If you decide to trust publishing to the third parties, you have to compromise.
Taking info from the link in @Sathya's comment above, the answer is to use Mobypicture.
Their page News: your content is yours is very clear (my emphasis)