A logo appears on a checkerboard in its preview, but the pattern follows it onto your page. That is a painted background, not PNG transparency. Editors often use a checkerboard to represent empty space, and a flattened picture can contain the same pattern as visible pixels.
For a sample logo, a gray-and-white pattern that moves with the logo onto a dark page is part of the file's visible content. A genuine transparent area lets the page's background show through instead.
Look beyond the thumbnail
Open the image against a plain light background and then a dark one. Do the empty areas change with the background, or does the checkerboard stay visible? Inspect the space between letters as well as the outer border.
The PNG specification defines transparency information, including alpha values. Alpha describes opacity: a pixel can be opaque, partly transparent or fully transparent. It does not tell you whether the graphic is well prepared for a particular background.
Image Transparency Inspector can show alpha groups and light/dark previews for supported images. Its stated format, bit-depth and size limits matter. A refused file needs another suitable inspection workflow, rather than an assumption that it has no transparency. PNG inspection also reports supported nonzero RGB beneath fully transparent pixels. Those colors are a separate question from the visible checkerboard: fully transparent does not mean the stored RGB is zero. JPEG/WebP browser decoding cannot reliably expose the same hidden-color information here.
Transparent edges can still look wrong
A logo may have a transparent background and still show a pale fringe on a dark page. Check the edge pixels separately from the large empty areas. Partly transparent pixels contribute to the displayed edge, and their colors can affect how it looks against another background.
Do not treat a fringe as proof of one specific technical defect. The inspector's preview is a clue about appearance, not a complete explanation of how the file was created. The source design and export settings may need review.
Try the image at the size and background you intend to use. A tiny preview can hide an outline that becomes obvious in a larger header. Conversely, inspecting only at extreme zoom can exaggerate a detail that is invisible in the actual placement.
Choose an output that supports your intention
Image Format Converter can prepare supported formats, but a conversion does not identify and erase an unwanted checkerboard. A JPEG output also needs an opaque background because JPEG does not preserve an alpha channel.
If you have the source design, export it with the intended transparent background. If you only have a flattened image, use an appropriate editing process and review the edges carefully. Do not rename the extension and expect the pixel content to change.
Before handing the logo to someone else, provide a preview on the intended background alongside the actual image file. That makes it easier to catch both a painted background and a visible fringe before the graphic becomes part of a page or printed layout.