Why local image conversion is safer

What a converter site normally does

The file is posted to a server, converted there, served back, and may sit in storage or logs. Terms of service often reserve broad rights, and retention is rarely visible.

What in-browser conversion changes

The image is decoded by your own browser and re-encoded by your own CPU. No request carries the picture, so there is no server-side copy to leak, and nothing to delete afterwards.

What it does not fix

The page itself may still load from the internet and your network still sees the request for the page (not the file). Local conversion also cannot protect you from a compromised device or from the file already sitting in a cloud folder.

Practical checklist

Prefer tools that work offline or state clearly that files are not uploaded; check whether the page makes network requests while converting; and for high-value documents, disconnect from the network before converting.