you have to upload the picture to a repository, the commit and open the image. At this place you can see “Raw” button, which links to direct link of the picture. This link is possible to use in wiki.
I do not recommend uploading static assets for the Wiki into your repo. Instead consider using a service like Keybase for a GPG encrypted public folder and whitelist your Keybase domain via CSP in your Gitea installation—and link to your image from there so you’re not taking up space in your git repo.