Why does Firefox mess up my digital images?

I hadn't been aware that Firefox has the ability to do such a neat trick, but you're right, in the Advanced Preferences area, there's an option called "Resize large images to fit in the browser window", as shown here:

While we're looking at this preference screen, a few more observations:

• If your system performance supports it, I strongly recommend that you choose both autoscrolling and smooth scrolling, though if I had to choose only one, it'd be the latter, not the former.
• I really love the different choices for "Open links from other applications in" and find that for my workflow the option "a new tab in the most recent window" is ideal.
• One thing that I find distracting is how the content of a page shifts down when I go from one tab to multiple tabs in the window, so I'm also pleased that Firefox lets me always show the tab bar even if there's only a single tab, by clicking on the "Hide the tab bar when only one web site is open" that you can see on the bottom of the screen shot.

Let's scroll down a bit and see if there are any more useful preferences:

Now you can see the rest of the tab bar related preference settings. Since I like to open new tabs behind my current tab (it's a great way to fly through an RSS aggregator, among other things, saving interesting articles to read after the first pass is completed) I leave "Select new tabs opened from links" unselected, as you can see.

A complaint that lots of Apple Safari users have, and one I bet a lot of new Internet Explorer 7.0 users will complain about too, is having a bunch of tabs open and clicking on the wrong button, just to find that all the tabbed windows close without a warning, leaving you, well, in the dust, lamenting your lack of forethought not bookmarking everything. The good news is that Firefox can save you from exactly this kind of accident by simply selecting "Warn when closing multiple tabs." Highly recommended.

Finally, given the insecure digital world we live in, it's important that you check for updates with some frequency. On the other hand, the themes and extensions that I use are cosmetic or just amusing, so I don't really care if they're completely up to date, so sometimes I uncheck "My Extensions and Themes" in this box. Of course, it'd be really nice if there were "Check for Updates..." and "Ext & Theme Updates..." options in the Menu bar too, so I could once in a blue moon check without having to remember where that "Check Now" button is hidden, but that's not how Firefox is designed.

Like many other programs, there are lots of little tweaks you can make to the Firefox preferences to make it match your own usage patterns and interactive needs. I recommend you check and experiment with them all, but start with the "resizing images" option so that you're not going completely crazy with your browser, okay? :-)