It's not complicated. First, you upload your image to the server of this website.
Then, open the image from the website and copy its URL address.
And then, when creating a new topic, click the "insert image" button and copy the URL address of the uploaded image into the field that appears.
Thank you very much! I'll try it. And how do I upload it to the server? Is it somewhere separate from the map creation topic?)
And also, are there any requirements for the preview image, such as pixel size?
I remember that when uploading screenshots for viewing the map, you could upload them directly in the topic, and there were size requirements.