Howdy Picky -
You're not alone. If you're coming from another language, Prolog's endlessly baffling for a while.
You have to expect a bit of learning curve going from a Cessna to a rocket plane.
Well, yes, the library docs, like most docs, assume you have a basic grasp of the language. So yes, it's written for people who know 99% of what's going on and only need that particular 1% explained.
And it's an especially deep dive for Prolog, given how nifty Prolog is.
Questions like 'how do I start the debugger?' can be answered with
Hanging out on ##prolog on freenode.net
IRC is a good way to get quick answers to those 'hey, it seems stuck' type messages.
There's no native image processing library. Shelling out to imagemagick or using the FFI
(foreign function interface) to bind to OpenCV C++ or using JPL and Java OpenCV bindings are reasonable options.
If you're going to be programming in Prolog I recommend getting 3 books.
Programming in Prolog with the ISO standard, by Clocksin and Mellish
The Art of Prolog, Leon Sperling
The Craft of Prolog, Richard O'Keefe
And if you want a 4th, I'd say 'The Practice of Prolog' and/or Ivan Bratko's book.
So, read LPN and/or Clocksin/Mellish, and figure out basics (yay, you've done that!)
Spend a while fiddling with SWI-Prolog and poking around in the website.
Read Art of Prolog to beef up your theory muscles.
Read the 7.0 release notes. Read the section on modules on the website.
if you want debogglized about the web framework, a good way to transition from toy programs to real software. (bias warning, I'm the author).
Prowl through the 'Add-Ons' link on website looking at various nifty packs
read some of the other tutorials on same website.
There's really too many libraries to explain them all. And they vary from the mundane and practical (SSL libraries) to the exotic (probablistic logic programming, which is nifty but boggling) and in between (the XPATH library is actually very cool).
Re the debugger - as a beginner, using the graphical debugger can often be more rewarding.
To start it query
on your predicate of interest, and it'll pop up a graphic debugger. More details in the FAQ document above.