Start by learning the basics of 9, and then you know where to look for solutions in more specialized topics in the user manual.
Here's a step by step example of how that process can work for you:
1) The Style Pane topic and Style/Formatting chapter are must reads for working with symbols and formatting. A handy list of topics in that chapter is here.
2) Based on what you read in the Style Panetopic, the topics that jump out as immediately relevant to using symbols, such as geological symbols, are Style: Points and Style: Bitmap Symbols. Read those.
3) Now you know how to use a vast range of sources for images. You don't say what form the symbols you have are in (.tif images? .png?), but quite likely you could use them directly.
4) Even better, it could be that you could use the sharper alternative of using symbol fonts. To find a useful geologic symbol font, this is what I did:
a) I used Bing to search for "geological symbol font" (don't use the quotes in Bing). Right away, there are many leads to what might be good fonts.
b) One of the links took me here, where there is a link to an ArcGIS 10 version. I clicked that link.
c) Not so interesting, but one of the comments took me to this page.
d) Perfect! That page has a link to the FGDCGeoSym and FGDCGeoAge fonts, both of which are in the public domain (use however you like) and can be installed on your Windows system and used by 9. Both are attached.
Those symbols are pretty good and will stay sharp, unlike bitmaps. I'd use those.