Basic and only output format from mapweaver itself is SVG. That has the advantage that all further formats contain all elements that mapweaver can produce. Disadvantage is a post-processing done by inkscape.
-out=file.svg (png and pdf names are automatic, DEFAULT=mapweaver.svg)
There are two options specifying additional output formats:
-png (also produce png, inkscape must be installed, very big) -pdf (also produce pdf, inkscape must be installed)
The names are automatic and derived from the -out name. Inkscape must be installed and your system must be able to run it from command line in the current directory. The PNG files are rather big. So maybe some post-processing is appropriate.
You can specify the resolution of the PNG file by using the -pngdpi parameter