Add images and an old template with image: 'logo.png' starts working. Nothing is uploaded anywhere — the files live in this tab's memory only.

For a custom font, add a .ttf and declare it in the template:
pdfMake.fonts = { Custom: { normal: 'MyFont.ttf', bold: 'MyFont.ttf', italics: 'MyFont.ttf', bolditalics: 'MyFont.ttf' } };
then defaultStyle: { font: 'Custom' }.