I have a SP2019 farm and SP Online. Using modern experience in both. In order to get custom app launcher tiles in the Sp2019, I ran the Hybrid Wizard and activated the Hybrid App launcher feature. I created two custom app launcher tiles in my M365 tenant and added pictures on them both. The Image is 60x60px large. I placed the picture in i Ducment library on a site in the tenant, and the used a shareable link. The link can be reached by "anyone", the edit checkbox is ticked.
The custom apps shows in both the SP2019 App launcher and SP Online. In SPO they show correctly with the chosen pictures, but in SP2019 the apps is showing without the pictures? It was about 12 hours i added the custom apps.