So as a conclusion (sort of):

After testing and researching, I have come to the conclusion that copy-pasting is not the best way to address this issue.

• Copy-paste implies that the pic is already exactly what you want (size, etc.) or then you need online tools to crop/resize.
• You don't see what you are pasting before you paste it
• Copy-paste implies that the pic is already in your clipboard. If you want to upload part of your desktop, you'll need a desktop tool to select part of your screen and copy.

Thus, it's much better to go the jCapture route, which is to select part of your screen and upload that.

jCapture is a nifty FOSS Java applet which is a plugin of DokuWiki:
http://www.dokuwiki.org/plugin:jcapture

The project leader was nice enough to add audio and video recording so we can now make screencasts! In effect, jCapture is AFAIK (And I did quite a bit of research on this), the World's first (and as of now only) web-based FOSS screen recording tool.

I hope more web apps and wiki engines will join us and integrate it. To help make this a reality, we have moved the project to:
http://sourceforge.net/projects/jcapture-applet/

If copy-paste is really what you still need, there is another DokuWiki plugin for this:
http://www.dokuwiki.org/plugin:supa

Best regards,

M 😉

References:
http://dev.tiki.org/Copy-Pasting+an+image
http://dev.tiki.org/jCapture
http://dev.tiki.org/ScreenCapture
http://dev.tiki.org/Screencast