Wait for marker image load

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**mapbox-gl-js version**: latest

### Question

Hi there !

i'm currently trying to get custom marker images to work in Svelte. It seems that sometimes (on refresh) the marker icons shows up and sometimes not, which is why i guess it could be loading issue.

This is my CustomMarker Component:

```

import { onMount } from "svelte";
import mapboxgl from "mapbox-gl";

export let marker: any;
export let map: any;

onMount(() => {
const markerElement = document.createElement("div");
markerElement.className = "custom-marker";

new mapboxgl.Marker(markerElement)
.setLngLat(marker.geometry.coordinates)
.setPopup(
new mapboxgl.Popup({ offset: 25 }).setHTML(
`<h3>${marker.properties.message}</h3><p>Test description</p>`
)
)
.addTo(map);
});

.custom-marker {
background-image: 'url("https://aqhnreobugwxvsogdiox.supabase.co/storage/v1/object/public/Images/example.png")';
background-size: cover;
cursor: pointer;
width: 30px;
height: 30px;
}

```

And this is how i add the markers on the map page:
```

The relevant entry points are the CustomMarker component and the map page's marker loop shown in the report; first reproduce the refresh-dependent behavior and compare the setup with the linked Mapbox Svelte tutorial. Done means the custom image marker is consistently rendered and clickable after refresh, with the cause and any required project change documented.

Written by the indexing model from the issue text.