I just downloaded the 1.5.1 "upgrade only." Is there any reason why it appears to include all sorts of unnecessary files? Entire sets of smilies, default avatars, logo.png (which would overwrite my own logo if not caught), etc., that surely are no different from 1.5. Am I missing something here?
It's actually the full package but with some key files, that shouldn't be overwritten, excluded.
The specific files you mention, shouldn't really be customised directly. There are Style Properties controlling various image paths that should be used to avoid default files overwriting your customisations.
You should have your logo elsewhere (e.g. another directory) and you should change the path of the logo to this in the ACP (in Header and Navigation) rather than replacing the default logo with your own. Doing it this way means your logo will never be overwritten when you upgrade.
@Chris D, @Martok, yes, understood on using a different name/path for the logo. Changing the subject to the Resource Manager, where this is no "upgrade only" version anyway, is there a similar solution for making sure I don't overwrite my own custom resource_icon.png file?
These simple steps, should get everything right for you before upgrading. I'm assuming your customised files are all currently in the styles/default directory:
Copy the styles/default directory to styles/YOURSTYLE
Go to the Admin CP
Ensure your style is selected and select the General group
Change "Path to Images" from "styles/default" to "styles/YOURSTYLE"
Obviously change YOURSTYLE to whatever you like.
Confirm everything is working properly, and you're still seeing your custom images. Now do the upgrade. Afterwards, check the images are still working. That should be problem solved. And, in future, just remember you need to customise the files in styles/YOURSTYLE and then you won't need to worry about them come upgrade time.