For a site, check Apple's download section of Automator actions or
Automator World
There's been a few summaries of it written and basic plug ins on the forums so it's probably worth a search with Automator in it.
You can only use the Apps and Actions in the list unless you install new ones which is where the site above come in. And yes, you're basically creating a mini-app or plug-in.
You need to remember the following - you need an object(s) for Automator to be working on and an action(s) to perform on them. Once you understand the basic structure of workflows then have a look at some of the test workflows that are already in Automator to see how they're built.
You can get your 'objects' which can be generic files/folders, image files, iCal events, bookmarks, URLs, music files from a variety of sources - they'll usually be represented in Automator by a 'Get x Files' or a 'Find x Files' or an 'Ask for x files' where x is an app from the list. Even if you're planning a plug-in where you'd highlight files in the Finder before rightclicking to run the Automator workflow, you'd still have a 'Get Finder Files' in there so that the workflow knows what its souce material is. Alternatively, if the files or folders are very specific, then you can specify it exactly in the Workflow as you build it.
Then you can drop the 'actions' that relate to what you want to do. The action will tell you what tupe of files it will work on - and what kind of files the action will export. You can click 'Run' at any point to check how things going.
Checking 'Options - Show as Run' in any action will mean that you don't have to be definite about what the workflow is doing. This means that you can change the destination folder, name of the file, size of the image etc, each time you run the app if you want to.
For your resize one, you don't need any extra actions. Preview has resize actions built in.
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If your original photos are in a folder in the Finder, your actions are going to be
Get Finder Files
Copy Finder Files to x (click on Options-Show as Run to select where x would be). This is only necessary if you want to save the originals. If you're happy to resize the originals, then you don't have to do this)
Resize images to x (click on Options-Show as Run if you want to specify the size/file format each time)
Then you save it as a plug-in (so that you can run it by rightclicking/Ctrl clicking on the file) or as an application that you can run or drop things onto.
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If your pictures are in iPhoto, then you'd do an 'Ask for iPhoto files' which brings up a dialogue box to allow you to select the images followed by the other steps above (though I'd definitely suggest a 'copy to x' in that case)
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If your next steps after resizing is always archiving them to a zip file and emailling them, you can then add actions to do that.
