[SWEngineer]

The difference is resolution is (apparently) due to compression on PentaxForums.
The difference in color (dull brown vs red-brown horse / vibrant vs dull green foliage) is all about color space, or more accurately color profile.
Here's what I observed and learned: In Safari (v 4.0), the Flickr image showed the proper color. In Firefox (v 3.0.13), both images had the dull brown color. Then I figured out how to turn on color management in Firefox.
(Thanks to davemdsm for pointing out that it could be done. No thanks to him not explaining how. [Just kidding Dave.] To save the rest of you the hassle - it isn't obvious at all - here's a link explaining your two options. I went with the plug-in option.
Rob Galbraith DPI: Firefox 3, released today, supports colour managed web browsing).

I went with the plug-in option. After restarting Firefox, both images had the correct color in Firefox. That sure came as a surprise. So I downloaded the individual images and discovered the Flickr file had sRGB as an embedded profile. The PentaxForums (PF) image file had no embedded profile at all. So now I'm curious if the OP uploaded a non-profile version to PF or if PF dropped the profile in addition to decreasing the resolution.
Regardless, here's what is going on: Safari uses the embedded color profile and so correctly displayed the Flickr version. But it apparently doesn't assume sRGB for the PF file. And since that file has no embedded profile, Safari did I-dont-know what to interpret/display the colors "incorrectly". Firefox (before I made it color profile aware) made the same "incorrect" guess for displaying both files. [I guess it and Safari are both handing off to the Mac OS thus the identical results.] Once I made Firefox color profile aware, it interprets the Flickr image correctly as expected but also (apparently) guesses correctly about the PF image file and so displays it correctly as well. How things work / appear on other systems (Windoz, Linux) and other browers, I can't say.

Bottom line to those posting photos - be sure to always convert to sRGB for posting and be sure you save the file with sRGB tagged as the embedded profile. Otherwise, all bets are off how your image will look, even in a web browser on your own system.

Trivia: the PREVIEW program that comes with Mac OS behaves the same as Safari regarding how it displays the Flickr vs. PF file. And if you want to test if your web browser correctly honors embedded color profiles, check this link:
Is your system ICC Version 4 ready?. I used it to verify Firefox color profile awareness was off and got turned on.