This is what Xenforo posted on their forum about resizing. I haven't had any trouble uploading anything with this platform where I did with the other. If you are getting errors it could also be the server. I suspect that would be interaction between server and xenforo in particular.
There's a hard limit on what can be resized and it defaults to 20 megapixels. An image which is 8618 x 3670 is a fairly whopping ~32 megapixels.
Certainly, if PHP tries to resize an image of those dimensions on some smaller servers, it would just cause an ugly out of memory error.
But it is configurable by adding the following to src/config.php:
$config['maxImageResizePixelCount'] = 20000000;
20000000 is the default value. For you to be able to upload that image, you'd likely need to increase it to 32000000 or even 40000000 if you envisage images of that size being uploaded frequently (gives a bit of breathing space for even larger images to be uploaded).
Once you have increased that limit, you will need to test to ensure there are no adverse effects, such as the aforementioned out of memory errors. If there are, you would need to have the memory_limit in PHP increased (depending on server hardware, you may even need a physical RAM upgrade to handle it).