My preference is PNG. This also saves transparency (alpha channel) in the image in the places where there is nothing in the scene to render. Useful if you want to superimpose a rendered figure or scene on a 2D background outside Poser.

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 I then tried to change the resolution to 300

Do you mean you changed the DPI setting to 300?
If this is what you meant, don't bother with it. DPI is only relevant for image printing processes, and makes no difference to exported image quality or file size.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)