It would be nice if the Krita builder started adding an icon to the AppImage. I have no idea how the build process works, though some basic Googling shows that it's done in the AppDir, read https://docs.appimage.org/reference/appdir.html#general-description for details.
Apparently this broke at some point. I have appimages in the 5.0 and early 5.1 series that have icon, as well as a 5.2 pre-alpha appimage with an icon, but recent builds don't have icons, so I can confirm this.
Hi there, this issue seems to have solved itself? All of a sudden my 5.1.5 AppImage seems to have the icon again.
I did a lot of research into the topic and it seems like the presence of an icon actually depends on the support from the file manager side. The AppImage runtime removed some legacy protocol near ~Krita 4.x release, so the icon has disappeared. I don't actually know if we can do anything about that problem.