I suggest using software that supports the EXIF standard - that way data you add to photos is preserved with the photo. All photo album software recognizes EXIF data. EXIF is pretty limited in what it can store, however. There is a standard called IPTC that goes even farther. And an emerging standard supported by Adobe called XMP. There’s an excellent program called iTag that will add this data to your photos. Here’s an email I received from the author of iTag:

Hi Leo,
I’m the author of iTag

I thought I’d write a quick email to give you a quick low down
on tagging your jpegs…so here it is.

There are 3 main headers one can use for metadata in jpegs - EXIF (as
you mentioned), IPTC and XMP. Hopefully by the end you will have seen
the light and will stop tagging your photos in Flickr and start using
IPTC instead :)

EXIF

As you know - populated by the camera and contains mostly properties
about how the image was taken - whether flash was on or off, exposure
time etc.
One thing that I find pretty cool is that EXIF can also store GPS
coordinates…so you can store not only the time your photo was taken
but also exactly where it was taken.
(Using a GPS with logging and marrying the timestamps of photo and GPS
logs makes this a breeze)

IPTC

This has been in use for many years by news organisations. IPTC has
many fields including
Copyright, Keywords, Caption, Caption Writer, Headline, City, Country
and a few more.
The news organisations invented IPTC headers for the same reason you
and I need it today - they had so much digital assets that they needed
a way to track them and to find them for say - pulling a ‘file photo’
when some news event happens.

I’m not sure when but consumers must have then started repurposing
IPTC for their own use. I actually had an email from an iTag user this
week who has been tagging his photos with IPTC for 5 years.
There are now quite a few applications that can read and write IPTC
headers in a jpeg. A small problem is that there are so many IPTC
fields and they are very news orientated that its not always clear
which field should be used to store the photo caption, keywords etc -
hence the ‘compatibility’ page on my site.
There seems to a bit of a defacto standard though and iTag is built to
be Flickr compatible…so you tag your photos once in iTag, upload
them to Flickr and voila, your photos in Flickr are already tagged.

XMP
---
There are some limitations with IPTC - keyword length limitations,
foreign language support (eg: Chinese characters) etc and so Adobe are
now pushing a newer standard - XMP - which is based on XML and is much
more flexible
(as opposed to the rigid, news orientated fields of IPTC).
I believe all of Adobe’s products now support XMP - and as an
indicator of the strength of the standard - Windows Vista supports XMP
via it’s Photo Gallery. Other applications are a little bit slower in
taking up XMP (including iTag).

So I hope that helps. It really is better to add all of your metadata
to your jpeg files and then upload to Flickr because so many other
apps can also use the data then.
Also you won’t be lost if Yahoo tanks and takes Flickr with it (I’m
sure its safe as houses, but a lifetime of photo tagging is a lot to
bet on Flickr hanging around :)

cheers
iTagger

PS: I’m a long time listener to TWIT & Security Now (and FLOSS too
although it seems on hiatus at the moment) - keep up the great work!