Sending objects from Jinja Templates to Python
Why do this? Any logic that you implement in the template is accessible to you in the controller of your app, including any variables that you place in the template context. 
If the data has been changed due to interaction with the user, then the best way to retrieve data, in my opinion, is to set up a form and use the normal POST method to send the request and the required data, correctly encoded and escaped, back to your program. In this way, you are protected from XSS issues, among other inconveniences. I would never do any processing in a template, and only use any local logic to modify the presentation itself.
EDIT Taking into account your scenario, I suggest the following:

User presses a button on a page and invokes a Get handler 
Get handler queries a database and receives a list of images the list is cached, maybe in a memcache and the key is sent with the list of images encoded as a parameter in the GET URL displayed by the template
List of images get passed to the template engine for display 
Another button is pressed and a different Get handler is invoked using the key received encoded in the GET URL, after sanitising and validation, to retrieve the cached list

If you don't want the intermediate step of caching a key-value pair, you may want to encode the whole list in the GET URL, and the step of sanitising and validation should be as easy on the whole list as on a key to the list. Both methods avoid a round trip to the database, protect you from malicious use, and respect the separation of data, presentation, and logic.
I started using Jinja Templating with Python to develop web apps.  With Jinja, I am able to send objects from my Python code to my index.html, but is it possible to receive objects from my index.html to my Python code?  For example, passing a list back and forth.  If so, do you have any examples?
Thank You!