With the private message (envelope), you can send it to just one person, or you can also sent it to multiple people - sort of like an "invite." It is a private message to the forum, but if the person or persons it's sent to are not currently on the forum, it will also send it to their email address they used when they signed up. They can either respond to it through the email, or they can enter the forum and respond that way. If you should so desire, you can also "invite" others to join in on the private conversation - since it was you who initiated the message. All of this will show up once you start the message process.
As far as being reluctant to show anything you've spotted (or think you've spotted), don't be. Many times by posting what you've seen (or think you've seen), others can look and see first-hand an example of something they may not have seen before. And even in cases where it isn't what it appeared to be, that is also part of the learning curve. Not everyone has had experience with satellite photographs before joining Tomnod. And as an added bonus, many times we find something - a cloud, a wave, a rock formation, etc. - that looks like a face, an animal, bird, fish, UFO, person, you name it. We post them under the "Fun FInds" or the "Campaign Specific" threads. While I admit the "Campaign Specific" thread is supposed to be for questions about the campaign and objects found that you have a question about, I have posted many "Fun Finds" objects there as well.
I agree with you about the differences in forum structures from one to another. I guess that comes with what the forum is used for and the results they were looking to achieve. So, just plug along and things will come to you in time.
Okay, got to go burp the baby! Grandma can't seem to get that last one out!