TB54 I think I get it now (sorry, I'm a dummy). So you have a custom welcome image HTML with some CSS transitions?
If you have full control over the image (e. g.: you added custom HTML through an extension, such as custom header HTML), then you might be over-complicating this - instead, look for a way to improve the CSS transitions in the HTML/CSS, so they don't trigger while the page is loading.
FYI this isn't very related to FreeFlarum. But if you insist, the following should work:
Custom header:
<style id="tempstyle">
html * {
transition: none !important;
-webkit-transition: none !important;
-moz-transition: none !important;
-ms-transition: none !important;
-o-transition: none !important;
}
</style>
<script>
window.onload = () => {
document.getElementById("tempstyle").remove();
}
</script>
