TIFF as a graphics format is not supported by pdflatex. This means that we can save the figures in TIFF format but we also need to save them in a format that pdflatex can handle (PDF, PNG, or JPEG).
We should note that using TIFF instead of PNG will slow rendering, especially with high resolutions.
Also, in the introduction we claim that
The result of this processing step is a pure markdown document containing only a YAML header, prose in markdown (and optionally LaTeX) syntax and rendered images in PDF, EPS, PNG, and TIFF formats at 300 dpi as set by apa6_pdf() or apa6_word(), respectively.
TIFF as a graphics format is not supported by pdflatex. This means that we can save the figures in TIFF format but we also need to save them in a format that pdflatex can handle (PDF, PNG, or JPEG).
We should note that using TIFF instead of PNG will slow rendering, especially with high resolutions.
Also, in the introduction we claim that
It's only PDF and PNG at this point.