Ok sorry I am a bit annoyed by people who really do not care to read anything, it happens quite a lot. I with they could close this entire ARVE subforum down. (technically I think its now even that, just a poor workaround)
Quick info because you not seem to know: WordPress is a software with a big rich company Automattic behind it. They run wordpress.com and sell stuff there. This is wp.org and is a community site and only for the open source software, they released to the public (and support it by paying the lead devs …). This is just a single Plugin coded for this software by a single person, me. The community is not really active in this subforum, more in the general forums about WordPress itself.
Solution to your Problem:
You said you looked hours, did you check the docs for ARVE? ??
So if you look at the ‘thumbnail’ attribute in the documentation This is easy: just thumbnail="https://your.image.url"
problem is you need the pro addon and the current dev version for it. If the install instructions are to complicated for you then you may want to wait till it becomes available here and the steps will be reduced. I can give it to you for free if you not already have the discount code on your setting page, if you want that let me know on my support site and I send it to you via email.