HEIF pictures

In Ubuntu 20.04: you have to install the package heif-gdk-pixbuf (no PPA needed):

In a terminal: sudo apt install heif-gdk-pixbuf

This will install libheif1 and libde265-0 along.

As of May 2020, programs that support HEIF via libheif1 include:

GNOME Image Viewer (Eye of GNOME - eog) is updated in Ubuntu 19.10 and later.

GPicView version 0.2.5-3

GIMP 2.10.2+

Krita version 4.2

gImageReader (in Ubuntu 20.04)

heif-thumbnailer - a thumbnailer for HEIF images that can be used by Nautilus and Nemo.

libheif-examples – provides command-line utilities: heif-convert and heif-enc.

Google Photos and Dropbox both reportedly support HEIF.
There are other online converters (e.g. HEICtoJPEG), but check their privacy policies before use.

Imagemagick, as of 7.0.7-22, can be compiled with the --with-libheif flag.
tifig is described as "a fast HEIF image converter aimed at thumbnailing". However, it must currently be compiled from source.
Nokia HEIF Info and Source Code. However, Apple's implementation differs somewhat.