All plugin icons should be SVG (PNG icons are deprecated)

[SingingBush]

Since Intellij 2018.2 SVG icons are supported and png is deprecated:

https://plugins.jetbrains.com/docs/intellij/work-with-icons-and-images.html#png-format-deprecated

Note that while considering updates to the existing icons it may also be a good time to make use of com.intellij.ui.LayeredIcon and com.intellij.ui.AnimatedIcon for some use cases to improve the user experience.

The com.intellij.ui.AnimatedIcon will be handy for some tooling related UI

AnimatedIcon(
    AnimatedIcon.Default.DELAY,
    com.package.icons.MyIconFrame1,
    com.package.icons.MyIconFrame2,
    com.package.icons.MyIconFrame3
)

The com.intellij.ui.LayeredIcon may be good for indicating different d file types (*.d, *.di, package.d currently all have different icons)

LayeredIcon.create(
    com.package.icons.MyIconLayer1,
    com.package.icons.MyIconLayer2
)