Let us clear up the most common misconception first. "Save a photo with a white background as a PNG and the background becomes transparent" is simply not true. Convert to PNG and that white background stays white. Changing the format and erasing the background are two completely different jobs.
Transparency is a capability, not content
PNG is a container that can hold transparency (an alpha channel). But just because the container can hold transparency does not mean any image you drop in becomes see-through. The source image has to actually be empty in those areas for that transparency to be preserved inside the PNG. Put a photo full of white pixels into a PNG and the container can hold transparency, but what is inside is still white.
Think of PNG as a clear glass bottle and a transparent background as empty space inside it. You can prepare an empty bottle, but pouring a full bottle into a glass one does not make the water disappear.
A JPG source has no transparency to begin with
This is even clearer when the source is a JPG. JPG has no alpha channel at all, so every pixel is opaque. In other words, there is no "transparent information" stored in the file. Convert that JPG to PNG and you get a great, lossless base for editing, but transparency that never existed is not going to appear out of nowhere.
So the real benefit of a JPG-to-PNG conversion is not transparency. It is that you can re-save the file repeatedly during editing and compositing without degrading quality.
How to actually get a transparent background
If you want a transparent background, order matters. Erase the background first, then save as PNG.
- Use an image editor or background-removal tool to actually delete the background pixels (a checkerboard pattern means it is truly transparent).
- Save that cut-out state as PNG. Save it as JPG here and the transparency gets filled back in with white or black.
- If you are already working with a PNG that has its background removed, only convert or resize while keeping the format so the transparency survives.
When you need to remove transparency (flattening)
When you hand a transparent PNG to a print shop or upload it somewhere with a fixed white background, removing the transparency is often the safer choice. Filling transparent areas with a specific background color to make one flat image is called flattening. Converting a PNG to JPG drops the alpha channel and fills the transparent parts automatically, so knowing in advance which color will fill them helps you avoid an unexpected black fringe.
In short, transparency does not appear because you changed the format; you get it by erasing the background and preserving it as a PNG. Switching's JPG to PNG converter processes files entirely in your browser and never uploads them to a server, so you can safely prepare a lossless PNG master before editing, or flatten a PNG to JPG, without any privacy concerns.