Display all original images automatically: no need to click the medium image. Works with the new pixiv layout (as of 2018-06).
There's 2 user options that can be easily changed in the code: - Fit images to screen width AND height. Disabled by default. - Load master images (low-res but faster) instead of original images (hi-res but slower). Disabled by default. If images take a long time to load, try enabling master images.
ALERT This script may not receive updates. We created a new extension, called Mini Pixiv. It works even better and does not require Tampermonkey/Greasemonkey.