Agreed, it does not make much sense to treat PDF and image formats differently.
I’d suggest to unify the dialog, no matter if pdf, jpg, png,…
Page size
Portrait/landscape
Width, height.
Resolution=dpi
…as parameters that are interdependent, and update accordingly in the UI, should a counterpart be changed.
Something else, related:
Setting the output format to image is not remembered in the dialog (R7). It always falls back to PDF.