Here are answers to your queries:-
1. In an email, an editable section can have only one snippet. Where as in a landing page, it is possible to have multiple snippets inside one editable scetion.
2. Yes it is possible to edit the image inside an existing snippet. That snippet has to be edited in Design Studio first and after changes are made, that snippet needs to be approved before using/ dragging it.
Let me know if you have further questions.
Do you add any wait time/ run into any issues dynamically populating content on a TY page based on it fitting into a segmentation defined by name / email? I notice that the page immediately does not load correctly but on refresh will have the right dynamic value displayed. Curious if there's a common fix since the value displayed on the TY page is linked to the form response.