I wrote the following for the
iWork Flickr Pool. I think it addresses your problem (which first-time users commonly encounter):
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Pages has two simultaneous ways of working — as a word processor, in which your text flows down the page and onto the next page — and as a layout app, which lets you position text and objects wherever you want.
If you're starting out, the trick is this: if your cursor is in the "text flow area" when you drag on an image or insert a shape, those items will go into the regular flow of your page, word processor-style. They will be "stuck" in place relative to the text around them. You can easily see where this text flow area is if you turn on the rulers.
But if you click outside the text flow area (say, on the margin somewhere), the blinking cursor disappears, and you can drag images or insert objects anywhere on the page. This is the mode I usually use; I can create new text boxes and put them wherever I want. Pages makes aligning and copying objects really easy.