Copyright/fair-use of other people's images

Would it be considered a copyright violation if someone were to lift an
image from a photographer's commercial web gallery, upload that image to
another hosting service (Tinypic.com or Pbase, for examples), and post the
new URL with a negative review of that image - all of this, without any
knowledge or consent of the photographer? No intention to profit.

Pbase has a clear policy that you should not upload any images to their site
if you are not the copyright owner for that image, IIRC. Tinypic.com, OTOH,
does not go into very much detail on this issue in their TOS.

Other scenario:

Can you lift a photo of a politician from a government site, alter the image
in a negative way (such as the George W Bush mock images with white powder
under his nose), and post it on your own website? Is this legal? Again, no
intention to profit.

Do different jurisdictions have different ways of dealing with these issues,
or is there a universal standard?

Would someone please explain "fair use" in the context of publishing other
people's images on the web? I've read the statutes. Can someone please
break it down?

Thanks in advance. For the record, the examples above are strictly
hypothetical scenarios. I have no plans to do any of those things.