I have created a tool that uses various unicode block graphics characters to improve the resolution (compared to the canonical 2 pixel per characterapproach taken by other solutions posted here). It also automatically displays images in a thumbnail mode if multiple images are provided as input.
Installation:
git clone https://github.com/stefanhaustein/TerminalImageViewer.git cd TerminalImageViewer/src/main/cpp make sudo make install
Usage:
tiv <image(s)>
More details at the github project: https://github.com/stefanhaustein/TerminalImageViewer
Example screenshot: