Easiest way to post pictures is to upload them to Imgur (imgur.com), then post them using the "Image" shortcut on the editor. It looks like a picture frame.
[background=rgb(255, 244, 228)]Your image must be hosted on a photo sharing website. Place the image code (of the actual image not the page, it will end with .jpg, .png, .gif) between the following code:[/background]
To answer your first question, any site really will do (Fotki, Photobucket, etc.) It's a case of personal preference, really.
As to the second, buying a membership certainly isn't required. Only two forum areas (both marked with the word "Extreme") are for paid-only members. Otherwise, the whole site is accessible. But I'm sure if you want to buy one, THP would be more than happy.