First, you should be add input type file at form and also add enctype='multipart/form-data' and

    <!-- -->
    <div class="field image">
        <label class="label" for="image"><span><?php /* @escapeNotVerified */ echo __('Image') ?></span></label>
        <div class="control">
            <input name="image" id="image" title="<?php /* @escapeNotVerified */ echo __('image') ?>"   type="file" />
        </div>
    </div>

At controller, you need upload image via

Magento\MediaStorage\Model\File\UploaderFactory

Add below class in __construct functions and execute function of controller:

 <?php
 namespace [Namespace];

use  Magento\Framework\App\Filesystem\DirectoryList;
class [MyClass]
{

 protected $uploaderFactory;
 protected $adapterFactory;
 protected $filesystem;

    public function  __construct(
            .....
            \Magento\MediaStorage\Model\File\UploaderFactory $uploaderFactory,
            \Magento\Framework\Image\AdapterFactory $adapterFactory,
            \Magento\Framework\Filesystem $filesystem
            .....
    )
    {
        .........
        $this->uploaderFactory = $uploaderFactory;
        $this->adapterFactory = $adapterFactory;
        $this->filesystem = $filesystem;
    }
     public function execute()
    {
        try{
            $uploaderFactory = $this->uploaderFactory->create(['fileId' => 'image']);
            $uploaderFactory->setAllowedExtensions(['jpg', 'jpeg', 'gif', 'png']);
            $imageAdapter = $this->adapterFactory->create();
            /* start of validated image */
            $uploaderFactory->addValidateCallback('custom_image_upload',
                $imageAdapter,'validateUploadFile');
            $uploaderFactory->setAllowRenameFiles(true);
            $uploaderFactory->setFilesDispersion(true);
            $mediaDirectory = $this->filesystem->getDirectoryRead(DirectoryList::MEDIA);
            $destinationPath = $mediaDirectory->getAbsolutePath('custom_image');
            $result = $uploaderFactory->save($destinationPath);
            if (!$result) {
                throw new LocalizedException(
                    __('File cannot be saved to path: $1', $destinationPath)
                );
            }
            /* you need yo save image 
                 $result['file'] at datbaseQQ 
            */
            $imagepath = $result['file'];
            //
        } catch (\Exception $e) {
        }
    }
}

Note

$destinationPath give the path till magentoDir/pub/media/custom_image and $result content below

I just have a suggestion for you. We can follow the same logic of product file option. We can see how Magento works with the file.

Here is a sample template:

vendor/magento/module-catalog/view/frontend/templates/product/view/options/type/file.phtml

And take a look at

vendor/magento/module-catalog/Model/Product/Type/AbstractType.php
public function processFileQueue() { ..... }

I think it's a good example for uploading image on frontend.