SVG icons not showing up in Qt4 release build in windows

for Qt 5.14 add a line to the .pro file

QT       += svg

Most likely you will have to include the plugins from your qt dir. This involves making a qt.conf file that is local to your app (see here: http://doc.qt.io/qt-4.8/qt-conf.html , more specifically the [Paths] section), and copy c:\Qt\4.x.x\plugins\imageformats*.dll to your distributable's directory.

For Qt5

Since Qt5 the framework has been heavily modularized (List of Modules). Most likely you are missing the svg module. The application will still compile without complaining. Make sure the SVG module is installed on your system and linked (with qmake (Howto), cmake (Howto) or plain make). If it was linked successfully QImageReader::supportedImageFormats() will list SVG.