Question One:
Pictures can be resized by adding a px measure -
imagename.jpg|100px
and can be framed too -
imagename.jpg|frame|My Caption
But our wiki refuses to resize AND frame a picture
imagename.jpg|100px|frame|My Caption
although the docs suggest it should work.
Does something need doing to our wiki to make this work?
Erh no. Forget about picture resizing (which is really your browser doing the resizing). Make your original picture the right size to begin with and all will be fine from there.
Question Two:
Can we open an external link in a new window (if so - how?) rather than over-writing the wiki window? Are there any opinions on whether this is a good thing or not? I always prefer it but some people consider it bad design to keep opening new windows.
If you mean wiki-links (the [[[My_Important_Topic_Type_Link]]]): no. If you mean external links, well hmmm... have you tried the old a href="blah" target="_blank"?
As for whether it is actually a good thing to open links in a new window: I used to be a fan of that, until I discovered the back-button which in combination with tabbed browsing really turns this into a moot discussion...

Question Three:
How can I get Safari working on my new Mac so I can be rid of this irritating Firefox thing which may render accurately but it sure ain't Mac software

Okay, so now you're scaring me. I've bought a new G5 (Tiger driven) which is to arrive in the coming week; but apparently this is giving you problems with Safari? Why does Safari not work in your present set-up? God forbid I have to turn to Firefox for surfing the web! (It's a Mac thing PC-people! Firefox hums along just fine on Windows!)