Hi Richard,
For the most part, "page" is not a term that you’d often see used in the context of Photoshop. That is, while you can use Photoshop to generate photo galleries for the web and other such purposes (well, with later versions than 3.0 that is), its primary use is an an image editor. A "page editor", to stay in similar terminology, would be an application such as Microsoft Word for general document publishing, or perhaps a web page editor such as GoLive, Dreamweaver, or FrontPage among others.
So, not being certain what you are referring to as a page, I’ll answer it in two ways:
1) If truly a page, as for the web or a Word document, you do not use Photoshop to replace one picture for another. Rather, you might use Photoshop to edit a photo to be used as the replacement, but the actual incorporation of the photo into the page would be a function of the page editor application you are using. For example, in Word you would use the Insert Picture From a File command, then browse to where your photo is saved and select it. Prior to that you would have cut out the original photo, usually just by clicking on it and doing a cut or delete operation.
2) If the "page" is actually an image file in and of itself, such as two photos side by side as a single image, then there are various ways you might go about replacing one of those photos with another. If the new photo is the same size as the old one, or perhaps even larger, then the process can be as simple as opening both the new photo and the old "page" of two photos, selecting the full area of the new photo, performing a copy, then moving to the "page" and pasting it into place there. I don’t recall if Photoshop 3.0 had layers yet or not, so I’ll not get into how they might be used. But, the general idea is that when you paste the photo into the new image, you should be able to position the pasted photo so it overlays the photo being replaced, and essentially hides that photo. Another approach is to first select the photo to be replaced, and cut it out of the "page" image. Then copy the new photo and paste it into the area where the prior image existed.
What you are attempting to do is a very basic and easy-to-learn operation in Photoshop. I suggest you just review the user’s manual for how to perform such operations. There may still be books on the subject, but most likely they will be for more current versions of Photoshop and the instructions may not necessarily apply to Photoshop 3.0 any more. Actually, moving up to a more current version of Photoshop might be a wise idea. Better still, if you’re just learning to use Photoshop for personal use only, then you’d likely find that Photoshop Elements 2.0 is more than suitable enough for your needs. It is sufficiently inexpensive that you could get software much more similar to the recent releases at Photoshop for a fraction of the cost (around $100 I think).
Hope that helps,
Daryl
