This is probably not a new 10.5 bug, now that I know the fix I can find mentions of similar problem from five years ago.
OS X 10.5 Leopard will, like XP's thumbnail view, show a preview of a JPG, PDF or other image file in place of the large icon.
In practice it's prone to losing the preview and just showing the generic icon. As per this discusison thread and an older thread that has the real advice the OS X launchServices database is fragile and often broken. (For example.)
One fix is to run a fairly complex terminal command that rebuilds that too fragile data source. Another is to download Onyx and run the rebuild launchServices and cleanup scripts (remove caches) then restart.
That restored my icon previews.
Another few hours lost to complexity collapse.
I'd really like to know why Apple hasn't either fixed this ancient bug or included a launchServices rebuild in the monthly maintenance script OS X runs.
I sent Onyx a $10US donation. I've used this app long enough to contribute something.