Briefly: display full-size screenshot images, side-by-side for multiple screenshots, but vertically with a click of the mouse button (because, depending on the images, one orientation is better for comparison than the other).

Less briefly: this is an update/companion to my http://freestyler.ws/style/4934/userstyles-org-screenshots-unresized-vertical style. Initially, this style will simply remove the sizing restriction placed on images--allowing them to display side-by-side, but at their native size. For multiple images this might mean they exceed the width of the window. To make them display vertically simply click on either of the images; when you release the mouse button, they'll return to the side-by-side display. To make the vertical display persistent simply click on either image and drag it a little until the "no drop" indicator (the circle with the line through it) appears and release the mouse button; when you're done, clicking anywhere on the page will restore the original side-by-side display. (I've left code in the style--commented out--which will restrict the display to the window width (100%) on hover; feel free to uncomment it, play with it, reverse the actions, etc. as you wish. I generally don't like such resizing because it slows down scrolling/paging; I prefer using the Javascript bookmarklet described in my aforementioned style when I want to "reduce" image(s), or just maximizing the window is enough to see all of the image. Also, you may prefer to uncomment the ':hover' for the initial display so that you only get the full-size image on hover; it's up to you.)

Demo time...
After installing this style, you can test it out using the following styles (but don't forget to disable any style with a similar "unresizing" function in order to get accurate results):

Better comparison with side-by-side display (default--no further action required*):
http://freestyler.ws/style/124385/firefox3-small-menubar-mod

Better view with vertical display (no probable need to scroll*--simple click & release):
http://freestyler.ws/style/4934/userstyles-org-screenshots-unresized-vertical

Better comparison with vertical display (possible need to scroll*--click & drag useful):
http://freestyler.ws/style/2464/google-reader-larger-and-serif-plus-wider

Large-ish image (may need to maximize Firefox window* [or reduce image using either the Javascript bookmarklet or some other method, or just mod this style to limit width to 100%] to see entire image):
http://freestyler.ws/style/11698/gmail-hide-some-hi-lite-some

* Depending on your window size