The AppImage needs to have a .desktop file in ~/.local/share/applications in order to show in the applications menu and activities overview search. Once you have started it from either of these locations, you should be able to pin it to the launcher. You may have to create the .desktop file yourself, and you may have to supply an icon.
Some AppImages will create a .desktop file, in ~/.local/share/applications, but most don't. For example, Etcher made a .desktop file on first run by double-click. It also automatically provided the icon, so it was easy. Krita didn't do either, as I recall, so I made the .desktop file, and had to find an icon for it on google. You should use an .svg file for the icon.
For example, here is a .desktop file for Krita, (note: my location for all my appimages is ~/appimages):
Code:
[Desktop Entry]
Version=1.0
Terminal=false
Icon=krita
Type=Application
Categories=Graphics;
Exec=/home/dmn/appimages/krita-4.2.5-x86_64.appimage %U
Name=Krita AppImage
GenericName=Graphics Design
StartupNotify=true
Save it as krita.desktop in ~/.local/share/applications
The Krita icon (krita.svg) was put in /usr/share/icons, which is a standard searched location.
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