You can make screenshots of any active vvvv-window by pressing the shortcuts Ctrl+1 (includes window border) or Ctrl+2. If you shoot a patch, the .jpg is placed in the same directory as this patchs .v4p file is in. If you shoot any other vvvv-window the .jpg is placed in the directory in which its parent patch is placed in (e.g. shoot a renderer and search the .jpg in the directory of the patch in which the Renderer node is placed in).
And if you would like to post a screensot directly to the "Screenshots of the Days" on vvvv.org, simply press CTRL+3.

If a patch is not yet saved (shows something linke 0.v4p//// in its windowtitle) you'll find the the .jpg in vvvvs root directory.
If you want to specify a directory where the screenshot is to be placed press Shift+Ctrl+1 (or Shift+Ctrl+2) to invoke a file-save dialog.
Pressing one of the shortcuts also copies the image to the clipboard which allows you to paste (Ctrl+V) it into other programs like Photoshop, Word etc.