In short: don't use the default Q2A function that lists questions. Do a MySQL query and list questions your own way. The description can be the raw content of the question content, or you can manipulate, shorten, etc. The image can be extracted by using PHP tricks.
In long: In my own Q2A installation, the theme developer overrides the function q_list($q_list).
It's very messy, and I also made a lot more customization such as displaying videos, so I will not copy here.
You can follow this https://nguyenkhaclong.com/29/show-first-image-as-thumbnail-in-the-question-list/ for the pure purpose of your question title.
If you are confident with PHP language, you can even make it simpler, just like my previous post
I'm just too lazy to redesign.
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