I'll try to answer a couple of your questions.
Postimage is a good site for photo hosting. Several people use it. Also Google Drive is used by a few. I use Flickr, though its a bit confusing to use and only allows a certain number of images to be hosted (though there's ways around that).
About build threads, it's a general term. Your Titanic model will go under "Ships", where you can start a "Topic" thread. You should see a blurb under the "Ships" topic for "Create a New Discussion Topic". This would be what is called a build thread. Try to indicate in the Title what the model is, scale, manufacturer and that it is either a work in progress or you are merely showing the finished build.
I hope that will be of some use.
Gary