I've been playing around with this a lot myself lately. There's a few things.
Firstly, your call should be : (note the 'of' in phpthumbof)
[[phpthumbof? &input=`[[DocInfo? &docid=`5` &field=`banner02`]]` &options=`w=420,h=300,zc=1,q=85`]]
But I should mention, I think (but not sure) phpthumbof is no longer supported, it's been forked as pThumb, which is updated regularly and fixes some bugs with phpthumbof. I was working around the zc bug with phpthumbof for months before I realised the existence of pThumb.
Next, I've never used DocInfo, I think GetResourceField does the same thing? just in case docinfo gives you grief.
And finally, I'll include a shoutout for
http://mun.ee"" target="_blank" rel="nofollow">Munee which I discovered a few weeks ago. Installation is a snap. Unpack it, create munee.php, add the rewrite to your .htaccess. Then your calls in modx would look like this:
<img src="[[DocInfo? &docid=`5` &field=`banner01`]]?resize=w[420]h[300]e[true]q[85]" alt="" width="420" height="300" />
When the browser requests the url, it's rewritten to munee.php, which resizes, caches, et cetera.
Why do I recommend it?
- it's quick, does the same thing as phpthumbof, but I'm yet to find any issues
- it handles images, css (& less/sass), and js (& coffee script); it compiles, catenates, minifies
Good luck!