Dashboard - custom icons

Some time ago, I created several custom dashboards where I could load my own custom icons for the individual displays.

Now I wanted to modify one of these dashboards and load a custom icon again for a new display (PNG/JPG format, 512x512 px) – but I briefly get an error message saying "Operation unsuccessful."

Have the parameters changed over the years?

Could you please reiterate the parameters and requirements for loading custom icons?

(Even though I'm still using version 3.70.18 and, at my age (73), won't be upgrading to version 4.)

Thank you!