How do I use two different images, one as a thumbnail and one as a linked image?

Unfortunately, WP seems completely incapable of doing this……

A lit bit sensational? Just because you don’t know how to do it doesn’t mean WordPress cannot do it.

First many (if not most) themes do this by default, when you insert your image into a post ( in media gallery) you can choose thumbnails which will link to the larger image versions. By default there are 3 sizes, you can add more.

http://codex.wordpress.org/Using_Images

You should look at these links for more advanced use for starters, but I have my doubts you will understand this:

• http://codex.wordpress.org/Post_Thumbnails
• http://codex.wordpress.org/Function_Reference/add_image_size
• http://codex.wordpress.org/Function_Reference/set_post_thumbnail_size

WordPress uses MySQL, your blog posts are stored in a database not a directory…your images are stores in the /uploads folder based on date, you can change this setting in your admin.