Correct problems with wordpress

General information for the website: eLearning website
Kind of development: Customization of existing website
Description of requirements/features: I'm having a few problems with my wordpress website. I have already turned off all pugins and changed the themes and it has not corrected the problems. I need someone who can solve these problems for me:
-Some of the Media files, which previously worked, are now leading to pages with just letters and numbers. Many of them were jpg or pdf scans.
-I use "elegant themes" which has a shortcode for an author box with has thumbnails. Those thumbnails no longer work, while they did previously. Instead I get an empty box where the picture should be.
- I need someone to create a "Child Theme" for me as I will be moving to a new theme soon.
- show me how to set up a button if people want to sign up for my RSS feed. I would only like some post categories to be listed in the RSS feed, however, since some of the posts that I create are for members only.
- Help me to identify images in my media library that may be too large and that may slow my site down.
-my website is www.orchardpeople.com.
Extra notes: