Magento 2 : Image upload using Graphql mutation

First convert your image into base64 then use that base64 string in to graphql input.

<?php

namespace VendoreName\ModuleName\Model\Resolver;

use Magento\Framework\GraphQl\Config\Element\Field;
use Magento\Framework\GraphQl\Exception\GraphQlInputException;
use Magento\Framework\GraphQl\Query\ResolverInterface;
use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
use Magento\Framework\App\Filesystem\DirectoryList;
use Magento\Framework\Filesystem;
use Magento\Framework\Filesystem\Driver\File;

class UploadImage implements ResolverInterface
{
    public function __construct(
        ....................................
        Filesystem $fileSystem,
        File $fileDriver,
        ....................................
    ) {
        ....................................
        $this->fileSystem = $fileSystem;
        $this->fileDriver = $fileDriver;
        ....................................
    }
    public function resolve(
        Field $field,
        $context,
        ResolveInfo $info,
        array $value = null,
        array $args = null
    ) {
        //your logic
        $this->uploadFile($fileData);
        //your logic
    }

    public function uploadFile($fileData)
    {
        // convert base64 string to image and save as file on server.
        $uploadedFileName = "";
        $fileName = '';
        if (isset($fileData['name'])) {
            $fileName = $fileData['name'];
        } else {
            $fileName = rand() . time();
        }
        if (isset($fileData['filecontent'])) {
            $mediaPath = $this->fileSystem->getDirectoryRead(DirectoryList::MEDIA)->getAbsolutePath();
            $originalPath = 'ModuleName/Attachments/';
            $mediaFullPath = $mediaPath . $originalPath;
            if (!file_exists($mediaFullPath)) {
                mkdir($mediaFullPath, 0775, true);
            }
            /* Check File is exist or not */
            $fullFilepath = $mediaFullPath . $fileName;
            if ($this->fileDriver->isExists($fullFilepath)) {
                $fileName = rand() . time() . $fileName;
            }
            $fileContent = base64_decode($fileData['filecontent']);
            $savedFile = fopen($mediaFullPath . $fileName, "wb");
            fwrite($savedFile, $fileContent);
            fclose($savedFile);
            $uploadedFileName = "/" . $fileName ;
        }
        return $uploadedFileName;
    }
}

Here $fileData is array and with $fileData['filecontent'] = base64 string of file and $fileData['name'] = file name.