WP Theme Showcase ext and i18n displays themes and info from their style.css with one or two indices. Lots of features and options and a widget.
WP Theme Showcase ext and i18n employs three different image sizes (if optimally configured; in the default options, 'screenshot.png' is used for all three image sizes). The showcase proper, which displays all the uploaded themes with a user-defined number of pieces of information (if available), makes use of medium-size screenshot files. Showcase ext creates one or two indices, "Table of Contents," that employ small-size screenshot files, "thumbnails." I have found that 100x100px images fit well in the thumbnail index.
If the user clicks the medium-size screenshot files, if optimally set up, he will be shown large pictures for a closer inspection of the design of the theme. If Shutter Reloaded or a compatible plugin is activated, the users may browse all the large pictures as a set. It is the responsibility of admin to provide these images. It is also possible to preview the theme with the content from the current blog.
If admin enters multiple user-defined filenames for the medium-size screenshots, Showcase ext will loop through these filenames and use the first filename that matches an existing file, as the theme image of the showcase proper. Optionally, it will add links to any additional image files found. If no screenshot is found, a placeholder is used.
Showcase ext can search for a number of readme filenames (the Options page provides a sample of such filenames, of which five are chosen upon first-time activation). You can modify the list in the Options. Showcase ext will create a link for each existing readme file, in the order of the filenames entered. This is useful, e.g., if translations of the readme files are available. Of course, the same feature can display links to license and changelog files (see my site for an example).
A later version may allow admin to enter additional directories for theme images (note that WP 2.9 supports this now; it should work with this version of the plugin as well) and readme files (thus allowing admin to keep his own files in a separate directory), especially if you tell me that you need it.