PNG image is blurry

First, PNG file format does have DPI embedded (DPI nomenclature is Microsoft's and industry standard, despite any issue you have with "dots" vs. "pixels").

You can determine the DPI from the file information using a free tool, such as IrfanView, which also enables you to alter that information.

Try the following:

• Open the file in IrfanView.
• Press letter I to view Image properties
• Change the default 72 x 72 DPI to improve it, perhaps to 300 x 300 DPI.
• Click the Change and then the OK buttons.
• Save the PNG image.
• Now try importing it into the LibreOffice suite -- Writer, Impress, or other application.

However, there may be a bug in LibreOffice concerning reading the embedded DPI from PNG images. If that is the case (though I have not observed that issue in my use of Writer), there are some things to try:

• Use IrfanView to change the PNG image to JPEG format, and set the DPI before saving the JPEG file.
• Alternatively, right-click on the image in Writer.
• Select Compress....
• Increase the resolution of the image.

Please let me know if any of these help create sharper images, and I'll edit the question to show what works for you.