Using your standard tool for copying files (e.g. Finder in Mac OS, or Explorer in Windows, or...), put the image file into the GitLab project. You can find the project as a regular folder in your files.
Go to your shell, and move into the folder of your Gitlab project. This usually means running the following command until you get to the desired destination:
Check if your image is actually present in the directory (if you are in Windows,
You should see the name of the image in the list shown.
Check the status:
Your image's name should appear in red, so
git took notice of it! Now add it
to the repository:
git add NAME-OF-YOUR-IMAGE
Check the status again, your image's name should have turned green:
git commit -m "DESCRIBE COMMIT IN A FEW WORDS"
Now you can push (send) your changes (in the branch NAME-OF-BRANCH) to GitLab (the git remote named 'origin'):
git push origin NAME-OF-BRANCH
Your image will be added to your branch in your repository in GitLab.