Any time I do a fresh install of the system, I find it necessary to change the default format of screenshots in Tiger to JPG from PNG.
Thanks to robg on MacOSXHints.com for this:

open Terminal and paste the following in. Replace image_format with jpg or whatever format you prefer.

defaults write com.apple.screencapture type image_format

You may have to kill the SystemUIServer in the Activity Monitor to get it to start using the new setting.