You’re right, F5 should be grabbing screenshots, but I do not see the screenshots/ dir that should be used. Steam’s overlay also supports screenshots, so you can use F12 for the time being.
Using the overlay screenshot key is much preferred here, you get the nice management UI and it automatically uploads to your account. But I can look at why the normal path fails.
The screenshots will go into ~/.steam/steam/SteamApps/common/Half-Life/, gamedir is "valve" or "cstrike" right now. Under OSX they are in ~/Library/Application support/Steam/SteamApps/common/Half-Life.
There is a bug causing them to go into the valve_hd or cstrike_hd folder (rather than valve/ or cstrike/), the next release will fix that.
Nothing extracted yet.
Not 100% sure here but.. i'm not able to save or retrieve screenshots while playing HL1,
i tested that multiple times, actually the F5 should acquire a new screenshot but
nothing found under my ~/Steam/SteamApps/