Per the file format list on the "Create" upload page, yes. (You could also use
VRML2 aka VRML97, but that is roughly the pre-xml dialect of the same format)
Yes, STL is triangles without colors or even units. (There is a potential successor named AMF that
adds both, but has not gained as much traction yet)
Yes, and the zip should have no internal folder structure, i.e. just the .x3d and png file.
Also make sure that the texture reference in the x3d file is just the filename without
any folder names.
And do not set any transparency on objects - while it probably does not matter for printing,
transparency is faithfully reproduced in the preview, which can be very confusing.