Is that PNG background white, gray or transparent?

A viewer can display transparent pixels over white, gray or a checkerboard. Those display colors are not necessarily part of the image. A PNG can also contain an opaque white background: the .png extension alone does not prove transparency.

Try these 128 × 96 examples. The transparent sample has clear outer pixels, an opaque blue upper rectangle and a half-transparent lower rectangle. The white-background copy is the same image flattened onto white. Switch this website’s color theme to compare their backgrounds.

Download the transparent input. In Photo, choose File → Open and select it. The editor uses a checkerboard behind transparent areas. Select Brush and paint a short black stroke inside the upper blue rectangle; leave the corners and lower-left part of the rectangle untouched.

If Photo cannot start with its default renderer, try the WebGL renderer.

Choose File → Export → Quick Export as PNG. Find the download, then use File → Open to reopen it. Check that it is still 128 × 96, your stroke is visible, the outer area is transparent and the lower rectangle still shows through to the backdrop.

For this sample we also inspected the downloaded pixels: untouched corners had alpha 0, the untouched lower rectangle had alpha 128, and the upper rectangle had alpha 255. Alpha 0 is transparent, 255 is opaque, and values between them are partly transparent.

Open the original and exported copy against the same backdrop. If both have a white background inside the canvas, the original may already contain white pixels. Exporting as PNG does not automatically remove a background. Avoid flattening onto a solid background when transparency matters, and keep the original.

Checked locally and on studio.borealbit.com with PhotoCraft 0.6 in Chrome using software WebGL. This small 8-bit sample does not establish background removal, general color-profile accuracy, layered PSD support or every PNG variant.