Welcome to the forums!
It's nice to see new people around here.

If you have played the training missions, I would appreciate some feedback about these missions, especially if there is something which might be a little weird or confusing.

To answer your question: Our forums do not have something to upload things, and there is no attachment feature, sadly. Instead, you have to upload your image somewhere else and just put a link to it.

You can either post the link directly. Or you can use the IMG tag (see https://hedgewars.org/filter/tips for details).

There are many websites to upload random files, one of them for example is:

https://ptpb.pw/f

Keep in mind that most of websites like these might eventually delete files to make room for new files.

If you want an image host that's easy to use and doesn't delete images, use Imgur. You can even CTRL+V images directly there and getting a proper link is a matter of 1-2 clicks.