Retrolium is described as 'Converts photos into pixel art' and is an app in the Photos & Graphics category. There are five alternatives to Retrolium for Windows, Linux, Mac and Windows S. The best alternative is PixelArtWorkshop. It's not free, so if you're looking for a free alternative, you could try SLK_img2pixel or Pixelator. Other great apps like Retrolium are PixaTool (Paid) and Pixelizer (Paid).
You can use PixaTool to get 8bit / Pixel style images or videos, optimize your PixelArt or game assets just adding some cool effects. Also PixaTool is able to convert images into ASCII/ANSI art exported in images(.PNG).
SLK_img2pixel is a tool for converting images to pixel art. It's not meant to replace an artist but rather assist in the creation of pixel art. It supports a variety of different features (different down-scaling, dithering, and color distance calculation modes).