The optimization patches in OpenComputers's repository are not yet merged into mainline, but you can install lunatic86 regardless! (Just with worse performance.)
$ oppm install lunatic86
$ lunatic86 -h
Apologies for scarce documentation, it's a bit of a project dump. It has a GUI, however. Feel free to fork and clean it up with the terms of the license in mind.
Those of you who have followed my OpenComputers work might be aware of CTIF, my high-quality image converter for OpenComputers. However, it has always required an external converter to pre-process image files. Others may be aware of greaser's ocpng, an utility-turned-library for displaying regular PNG files without pre-processing - however, that capped out at 160x50 as it only used low-resolution characters.
However, I am proud to present pngview - a .PNG viewer right in OpenComputers, based off greaser's ocpng library but capable of displaying images at up to 320x200. Unfortunately, it does not display images as well as CTIF - it has far more severe limitations related to being a simple Lua script that, as such, cannot perform calculations that complex. In particular, it handles slowly changing gradients a lot worse and is incapable of finding additional colors to supplement the image's palette the way CTIF does.
Comparison: the top image is viewed via pngview, the bottom image is rendered via CTIF.
Currently, it only works on Tier 3, as the palette calculation process relies primarily on the 240 RGB colors exclusive to it.
It does not perform any image scaling, so ~320x200 is the maximum image size (on Tier 3).
pngview is part of a Lua script collection called octagon.