Attach Bevy's Handles/Entities statically to Types.

use bevy_cell::*;
// setup cells
handle_cell!{Image: handle_a;}
entity_cell!{entity_a}
// on startup
Image::set_handle_a(img_handle);
Entity::set_entity_a(entity);
// in any system
Image::get_handle_a();
Entity::get_entity_a();

handle_cell!{
    Image: image_a, image_b, image_c;
    Mesh: mesh_a, mesh_b;
}
handle_cell!{ @Vec
    Image: image_vec_a, image_vec_b, image_vec_c;
    Mesh: mesh_vec_a, mesh_vec_b;
}
entity_cell!{
    entity_a, entity_b, entity_c
}

handle_cell!{@Vec Image: numbers;}
handle_cell!{@HashMap<K> Image: numbers_id;}
entity_cell!{@Vec slots}
entity_cell!{@HashMap<K> slots_id}
// on startup
Image::set_numbers(img_handle_vec);
Image::set_numbers_id(img_handle_map);
Entity::set_slots(entity_vec);
Entity::set_slots_id(entity_map);
// in any system
Image::get_numbers(index);
Image::get_numbers_id(key);
Entity::get_slots(index);
Entity::get_slots_id(key);

use bevy_cell::*;
use bevy::prelude::*;

handle_cell!{Image: number;}
entity_cell!{slot}

fn startup (
    mut cmd: Commands,
    assets: Res<AssetServer>
){
    cmd.spawn(Camera2dBundle::default());
    // set Handles/Entities once on startup
    Image::set_number(assets.load("num_005.png"));
    Entity::set_slot(cmd.spawn(SpriteBundle{
        texture: assets.load("num_000.png"),
        ..default()
    }).id());
}

fn update (
    mut timer: ResMut<ChangeTimer>,
    time: Res<Time>,
    mut qu: Query<&mut Handle<Image>>
){
    timer.0.tick(time.delta());
    if timer.0.just_finished() {
        // get Handles/Entities anywhere
        *qu.get_mut(Entity::get_slot()).unwrap() = Image::get_number(); 
    }
}

#[derive(Resource)]
struct ChangeTimer(Timer);

fn main () {
    let mut app = App::new();
    app
    .add_plugins(DefaultPlugins)
    .add_systems(Startup,startup)
    .add_systems(Update,update)
    .insert_resource(ChangeTimer(Timer::from_seconds(1.,TimerMode::Once)))
    .run();
}

use bevy_cell::*;
use bevy::prelude::*;

handle_cell!{@Vec #load #random Image: numbers;}
entity_cell!{slot}

fn startup (
    mut cmd: Commands,
    assets: Res<AssetServer>
){
    cmd.spawn(Camera2dBundle::default());
    // loads num_000.png - num_009.png and attach it as Vec<Handle<Image>>
    // into the Image type. Image::get_numbers(index) returns the Handle now.
    Image::set_numbers((&assets,"num",0..=9,"png").into());
    // Saves the spawned Entity into the Entity type. Entity::get_slot()
    // returns this entity now anywhere.
    Entity::set_slot(cmd.spawn(SpriteBundle{
        texture: Image::get_numbers_random(),
        ..default()
    }).id());
}

fn update (
    mut timer: ResMut<ChangeTimer>,
    time: Res<Time>,
    mut qu: Query<&mut Handle<Image>>
){
    timer.0.tick(time.delta());
    if timer.0.just_finished() {
        // Change the Handle<Image> of the Slot-Entities to a random number-image.
        *qu.get_mut(Entity::get_slot()).unwrap() = Image::get_numbers_random(); 
    }
}

#[derive(Resource)]
struct ChangeTimer(Timer);

fn main () {
    let mut app = App::new();
    app
    .add_plugins(DefaultPlugins)
    .add_systems(Startup,startup)
    .add_systems(Update,update)
    .insert_resource(ChangeTimer(Timer::from_seconds(1.,TimerMode::Repeating)))
    .run();
}