so it seems there is no binding fopen() ↔ SPIFFS.open(). And if you just call fopen("icon.gif", "rb") file will not open. I don’t know how to configure PlatfomIO to link stdio with SPIFFS (if this is possible).
Maybe somebody will help with it.
Anyway you can use lv_fs - driver . In this case you need ro realize open(), close(), read() function which will use SPIFFS.open/close/read functions. Here, here, here some examples.