Privacy
Incognito hides your history, not your upload
A private window stops your own device remembering. It does nothing about the server on the other end.
Guides on Privacy: Three kinds of hidden labels in one file, Who could see it, how, and what each path costs to close, Soft delete, backups, caches and logs, Sometimes, and the policy clause that says so is easy to miss
Private browsing hides what you did from the next person who uses your device. It does not hide it from the website, from your network or from anyone who can see your connection. The upload goes to the server exactly as it would in a normal window.
What it clears
In a private window the browser keeps browsing data in temporary storage and discards it when the window closes. That covers history, cookies, form entries and cached files. The practical effect is local: a family member who opens your history afterwards finds nothing from that session. This is the first of two goals that Aggarwal, Bursztein, Jackson and Boneh (2010) identified in the browsers they studied, that sites visited in private mode should leave no trace on the user's computer. The second goal they named, hiding your identity from the sites you visit, is the one that is much weaker in practice.
So the thing it does well is what the cache and history left after an upload would otherwise leave on a shared device.
What it does not touch
The server on the other end sees an ordinary request. It receives your file, your IP address and the usual headers. It can log them, store the image and tie the upload to an account, exactly as before. Nothing about the window changes what the destination does.
Your network sees the connection too. A home router, a workplace network or a school network can usually record which sites you connected to, even if the content is encrypted. An employer who manages the device may also have software on it that watches browsing, and a private window does not switch that off. Your internet provider sees the destination in the same way.
Extensions and any login matter as well. Signing into an account in a private window tells the site who you are, and everything you then do is linked to that account. Browser extensions may run in private mode depending on settings, and the research above found that extensions can undermine the guarantees.
The common misreading
The word "private" suggests more than the feature delivers. It is a local-cleanup mode. People expect it to behave like a VPN, and it does not, which is why what a VPN does and does not hide is a separate question with its own limits. Even a VPN leaves metadata inside the file, account linkage and fingerprinting alone. Private mode leaves your IP address visible as well.
A useful way to hold it is by asking who the adversary is. If the worry is someone who picks up your phone or laptop later, private mode is relevant and cheap. If the worry is the service, a third-party script or anyone on the path, it is not the right tool. Fingerprinting techniques can also still recognise a browser inside a private window from its configuration, which cookies were never needed for.
Putting it to use
Used for the job it does, it is worth having. Open the window before the upload, close it afterwards, and the device keeps no record of the session. That sits alongside removing metadata from the file and checking what the service retains, rather than replacing either. If you are new to the whole area, cockrate.com's starter pages give the basics, and measuremycock.com's data section explains what a service might collect. For how to treat images you receive from other people, ratepenis.com on etiquette covers the human side of the same concern. Rate Cock can be used from a private window like any site, and the same limits apply to it.
What a private window cannot do is un-send anything. Once the file has left your device, the only question left is what the other end does with it.