Hi,

I'm trying to replace an icon but I can't see where it's stored.

The graphic I'd like to replace is the yellow + symbol here:

Does the yellow plus only appear when Chromium is running, or is it always present?
Also, does clicking on the yellow plus have any effect in Chromium?

It is quite likely this yellow plus is your user account avatar within Chromium.

The plus appears on *any* grouped set of windows. E.g. if I open two instances of Kate I'll get a plus on the launcher, if I open two instances of Konsole, I'll get the yellow plus on that Icon.

The plus icon, based on what I can tell from the code at least appears to be the SVG element "horizontal-dropindicator" or "vertical-dropindicator" (depending on the orientation of the applet). These come from the SVG / theme element called "icontasks/dropindicators".