Magento 2 Admin Form WYSIWYG Field Width Problem

It should work with value:

$fieldset->addField('register', 'button', array(
    'value' => Mage::helper('module')->__('Button Caption'),
    'onclick' => 'test()'
));

Just make sure that at the end of your form declaration you don't do this:

$form->setValues(....)

This will override the value of the button input. Use this instead.

$form->addValues(....)

For adding image field, you can try using plugin and always try to avoid overwrite whole class.

Vendor/Module/etc/adminhtml/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\User\Block\User\Edit\Tab\Main">
        <plugin name="sr_stackexchange_user_form" type="Vendor\Module\Plugin\Block\Adminhtml\User\Edit\Tab\Main" sortOrder="1"/>
    </type>
</config>

Vendor/Module/Plugin/Block/Adminhtml/User/Edit/Tab/Main.php

namespace Vendor\Module\Plugin\Block\Adminhtml\User\Edit\Tab;

class Main
{
    /**
     * Get form HTML
     *
     * @return string
     */
    public function aroundGetFormHtml(
        \Magento\User\Block\User\Edit\Tab\Main $subject,
        \Closure $proceed
    )
    {
        $form = $subject->getForm();
        if (is_object($form)) {
            $fieldset = $form->addFieldset('admin_user_image', ['legend' => __('User Image')]);
            $fieldset->addField(
                'user_image',
                'image',
                [
                    'name' => 'user_image',
                    'label' => __('Image'),
                    'id' => 'user_image',
                    'title' => __('Image'),
                    'required' => false,
                    'note' => 'Allow image type: jpg, jpeg, png'
                ]
            );

            $subject->setForm($form);
        }

        return $proceed();
    }
}

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