I don't know if Picasa has an email restriction, but Flickr's viewing restrictions either mean they're visible, or they're not, for the most part. If you don't want just anyone seeing them, the person you're sharing them with would have to also have an account, and you'd have to set your photo to be viewable by contacts only, which would be a pain for something like an approval process, or even just sharing family photos.
You could upload your photos to Dropbox. There, you can generate an item specific URL that you can then send to specific people. Without accessing that URL, your files are invisible to all but those that have administrative access to your Dropbox's root folder.
Edit: To include Twitter and FB in the mix, you could then make an image post on Twitter, and have your FB account automatically re-post your twitter updates, while your original file sits in your Dropbox. Bit of a Rube Goldberg solution, but it could get the job done.