A family of Microsoft relational database management systems designed for ease of use.
PNG files are images. Visio can import them as Visio "Shapes", but you can not edit the content of the shape.
If the file is SVG, you can ungroup and play with the component shapes.
How did you get the PNG file? Visio can save as PNG, but in doing so, you lose a lot of content. It may look right, but it is uneditable.