When searching for artwork on my computer is there anyway I can tag the file to include its contents?

I'll reserve snarky comments for the peanut gallery; If you using a mac, you can create tags in iphoto, place tags in the "spotlight" column if you "more info the file." Additionally, if you're using PC, I think picasa will facilitate the same thing.

i forgot to mention, i do use a mac (boots 9 and X) and have another computer running xp.

If you have any artwork in PDF format and you have Acrobat Pro, you can go to Advanced --> Document Metadata and then enter tags in the "keywords" field. These are stored in an embedded http://www.adobeopenoptions.com/products/xmp/ file.

JPEGs have a built-in metadata format called IPTC. Lots of smart programs that deal in JPEGs recognize this. Flickr recognizes it. Spotlight recognizes it. One of the fields in IPTC is "keyword", so if you've got the right software, you can get it in there. I use Graphic Converter, and it will handle this (it will also apply IPTC to tiffs and other file formats). This will travel with the file and could be considered the canonical, cross-platform approach. There's a program called SpotMeta for OS X that builds on Spotlight and lets you define your own metadata types (keyword would be just one). Not sure if this would transfer across Macs (much less platforms), but it would work on your Mac. Or, duh, you could do like AllesKlar herr commissar suggests and just use the Comments field in the Finder.

I would suggest http://www.adobe.com/products/photoshopelwin/. I use this to catalog all of my digital photos. It supports Tagging, and you can also write the tags to the photos if you ever wanted to use another program in the future. Also you can have totally separate catalogs, so in your example you could keep one for work and one for personal photos. It allows you to do just what you said - select a tag or tags (such as "tree") and it will show all of the photos tagged with tree. It's available for both PC and MacOS X.

You can do this quite easily in OS X (iPhoto or other software isn't needed either). Hit command-I to "get info" on any file in the Finder and type in whatever tags, keywords, or info you want in the "Spotlight Comments" area. When you search for something using Spotlight, that information gets searched too. This is, of course, if you have upgraded to Tiger (you mention your machine boots into 9, which means you probably got your Mac before Tiger was released)