How can I control dataScope?

If you want to control whole name of POST key (like prefix[name]) on Field level then you'll interested in discard autocomposing it.

First of all how it composed? When a Field placed in chain Form1 > Fieldset1 > Fieldset2 > Field, then Field gets POST key Form1[Fieldset1][Fieldset2][Field].

If you want to omit Fieldset1 from chain then you configure its dataScope as empty sting. Examples:

Fieldset1.dataScope = "" -> POST key is Form1[Fieldset2][Field].

Fieldset1.dataScope = ""
Fieldset2.dataScope = "" -> POST key is Form1[Field].

Form1.dataScope = "data"
Fieldset1.dataScope = ""
Fieldset2.dataScope = "" -> POST key is Field.

Form1.dataScope = "data.prefix"
Fieldset1.dataScope = ""
Fieldset2.dataScope = "" -> POST key is prefix[Field].

Form1.dataScope = "data"
Fieldset1.dataScope = ""
Fieldset2.dataScope = ""
Field.dataScope = "prefix.field2" -> POST key is prefix[field2].

What does dataScope affect?

You should remember that dataScope is not a POST key. It's a keychain from DataProvider object. In xml configuration there is <dataSource/> tag. This tag configures who is provider data for form. Client side app produces DataProvider object and link every Field's value with some DataProvider's key. So string dataScope just point to some DataProvider's key.

If you want to control Form's POST object it means that you want to control DataProvider's data structure also.

I found a way to do it without requiring a class attached to the field. I mean there is a class attached to the form element but not as a renderer.
The column should be defined as this:

<field name="image">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="dataType" xsi:type="string">string</item>
            <item name="source" xsi:type="string">[entity]</item>
            <item name="label" xsi:type="string" translate="true">Image</item>
            <item name="visible" xsi:type="boolean">true</item>
            <item name="formElement" xsi:type="string">fileUploader</item>
            <item name="elementTmpl" xsi:type="string">ui/form/element/uploader/uploader</item>
            <item name="previewTmpl" xsi:type="string">[Namespace]_[Module]/image-preview</item>
            <item name="required" xsi:type="boolean">false</item>
            <item name="uploaderConfig" xsi:type="array">
                <item name="url" xsi:type="url" path="[namespace_module]/[entity]_image/upload"/>
            </item>
        </item>
    </argument>
</field>

I also needed to create the preview template file referenced by [Namespace]_[Module]/image-preview.
That is app/code/[Namespace]/[Module]/view/adminhtml/web/template/image-preview.html that looks like this:

<div class="file-uploader-summary">
    <div class="file-uploader-preview">
        <a attr="href: $parent.getFilePreview($file)" target="_blank">
            <img
                class="preview-image"
                tabindex="0"
                event="load: $parent.onPreviewLoad.bind($parent)"
                attr="
                    src: $parent.getFilePreview($file),
                    alt: $file.name">
        </a>

        <div class="actions">
            <button
                type="button"
                class="action-remove"
                data-role="delete-button"
                attr="title: $t('Delete image')"
                click="$parent.removeFile.bind($parent, $file)">
                <span translate="'Delete image'"/>
            </button>
        </div>
    </div>

    <div class="file-uploader-filename" text="$file.name"/>
    <div class="file-uploader-meta">
        <text args="$file.previewWidth"/>x<text args="$file.previewHeight"/>
    </div>
</div>

This code will generate a field like this:

After uploading an image (real time) it looks like this:

The url item inside the uploaderConfig is the url where the image is posted when uploaded. So I needed to create this also:

namespace [Namespace]\[Module]\Controller\Adminhtml\[Entity]\Image;

use Magento\Framework\Controller\ResultFactory;

/**
 * Class Upload
 */
class Upload extends \Magento\Backend\App\Action
{
    /**
     * Image uploader
     *
     * @var \[Namespace]\[Module]\Model\ImageUploader
     */
    protected $imageUploader;

    /**
     * @param \Magento\Backend\App\Action\Context $context
     * @param \[Namespace]\[Module]\Model\ImageUploader $imageUploader
     */
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \[Namespace]\[Module]\Model\ImageUploader $imageUploader
    ) {
        parent::__construct($context);
        $this->imageUploader = $imageUploader;
    }

    /**
     * Check admin permissions for this controller
     *
     * @return boolean
     */
    protected function _isAllowed()
    {
        return $this->_authorization->isAllowed('[Namespace]_[Module]::[entity]');
    }

    /**
     * Upload file controller action
     *
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {
        try {
            $result = $this->imageUploader->saveFileToTmpDir('image');

            $result['cookie'] = [
                'name' => $this->_getSession()->getName(),
                'value' => $this->_getSession()->getSessionId(),
                'lifetime' => $this->_getSession()->getCookieLifetime(),
                'path' => $this->_getSession()->getCookiePath(),
                'domain' => $this->_getSession()->getCookieDomain(),
            ];
        } catch (\Exception $e) {
            $result = ['error' => $e->getMessage(), 'errorcode' => $e->getCode()];
        }
        return $this->resultFactory->create(ResultFactory::TYPE_JSON)->setData($result);
    }
}

This class uses an instance of [Namespace]\[Module]\Model\ImageUploader that is similar to \Magento\Catalog\Model\ImageUploader.

This seams to work. I still have troubles saving the image in the db but that's a totally different issue.
I used as inspiration the image field for the category entity