How to set my wallpaper from my Dropbox? I have copied a bunch of images into my Dropbox on my pc and sync'd them to my phone. Does anyone know how to use these images for wallpapers on my phone?

The only way I can think of is that I can use something like Astro to copy the file to my gallery locally on the phone, or to share my dropbox pictures folder on the web and browse to it.

Although copying the file into dropbox and then copying it again seems crazy. Is there a better option?
You're right I just played with trying to set a Dropbox image as my wallpaper, and can't believe that there isn't an easier way. Part of the problem seems to be that if you Open a dropbox image, it isn't sent to the Gallery or the system default image viewer (which would let you do things like set as wallpaper) but Dropbox uses its own (HTML?) viewer.

There are feature requests in Dropbox's "Votebox" that are requesting features that would fix this. It's probably worth heading over there and letting them know that you'd like this feature, this option looks like the most likely (but you can just go to Votebox and search for Android to see all the relevant ones):

Add Dropbox photos to Android gallery

Update: If you download photos from Dropbox, they do appear in the gallery in a folder called "Photos". Now that they're in the Gallery, you can set them as wallpaper the same as normal. You might need to force the media scanner to start before they'll show up in the gallery, this previous question has options for forcing that to run.