Greetings AnsjelaAmat✦, I see that you have shared a 360 degree picture. You should try to post that again, but instead of clicking on "Post to Region", click on "More...". And then click on "360 Snapshop". Next click on the "Choose File" button under "360 Image file", and select the 360 image. Then take another regular snapshot, and upload it at the second "Choose File" button under "Thumb Image". The regular snapshot will appear briefly when loading the post, and then the 360 image will appear rotating like this: https://opensimworld.com/post/126924 although you can click on the image to move it around.
Also, if you share your post link to the Discord group, the thumb image will appear. https://discord.gg/cKB3vGbu
Thank you, Shalom