So after some progress I discovered that Resources Importer in 7.1 does not import images if they are used in "advanced" structures and templates and it also creates side bug that if I want to create Site from template then pages are not created according to sitemap.json due to fact that theme contains structures and templates which contains image.

Replication:

1.) Create Liferay 7.1 Theme using Blade CLI Graddle wrapper in Liferay Workspace

2.) Create structure, template and webcontent

{
  "availableLanguageIds": [
    "en_US"
  ],
  "defaultLanguageId": "en_US",
  "fields": [
    {
      "label": {
        "en_US": "Image"
      },
      "predefinedValue": {
        "en_US": ""
      },
      "style": {
        "en_US": ""
      },
      "tip": {
        "en_US": ""
      },
      "dataType": "image",
      "fieldNamespace": "ddm",
      "indexType": "text",
      "localizable": true,
      "name": "Img",
      "readOnly": false,
      "repeatable": false,
      "required": false,
      "showLabel": true,
      "type": "ddm-image"
    }
  ]
}

<#if Img.getData()?? && Img.getData() != "">
    <img src="${Img.getData()}" />
</#if>

<?xml version="1.0"?>

<root>
    <dynamic-element name="Img" type="image" index-type="text" instance-id="mhto">
        <dynamic-content><![CDATA[[$FILE=1.png$]]]></dynamic-content>
    </dynamic-element>
</root>

3.) Create sitemap.json

{
  "layoutTemplateId": "1_column",
  "publicPages": [
    {
      "friendlyURL": "/home",
      "name": "Home",
      "title": "Home",
      "layoutTemplateId": "1_column",
      "columns": [
        [
          {
            "portletId": "com_liferay_journal_content_web_portlet_JournalContentPortlet",
            "portletPreferences": {
              "articleId": "Img.xml",
              "groupId": "${groupId}",
              "portletSetupShowBorders": "false"
            }
          }
        ]
      ]
    },
    {
      "friendlyURL": "/login",
      "name": "Login",
      "title": "Login",
      "layoutTemplateId": "1_column",
      "columns": [
        [
          {
            "portletId": "com_liferay_login_web_portlet_LoginPortlet",
            "portletPreferences": {
              "portletSetupShowBorders": "false"
            }
          }
        ]
      ]
    }
  ]
}

4.) build theme blade gw build

5.) deploy theme to Liferay 7.1 CE GA2

6.) Preview Site Template and see that there is no Image visible.

7.) create Site from Site Template and there are no pages created according to sitemap and there are no web content and documents imported

If I use same theme at Liferay 7.0 CE GA7 it works.

If I use just basic webcontent it works with 7.1:

Structure

{
    "availableLanguageIds": [
        "en_US"
    ],
    "defaultLanguageId": "en_US",
    "fields": [
        {
            "label": {
                "en_US": "Content"
            },
            "predefinedValue": {
                "en_US": ""
            },
            "style": {
                "en_US": ""
            },
            "tip": {
                "en_US": ""
            },
            "dataType": "html",
            "fieldNamespace": "ddm",
            "indexType": "keyword",
            "localizable": true,
            "name": "content",
            "readOnly": false,
            "repeatable": false,
            "required": false,
            "showLabel": true,
            "type": "ddm-text-html"
        }
    ]
}

Template

${content.getData()}

Web content

<?xml version="1.0"?>

<root available-locales="en_US" default-locale="en_US">
    <dynamic-element name="content" type="text_area" index-type="keyword" index="0">
        <dynamic-content language-id="en_US">
            <![CDATA[
<div>
    <strong>Copyright</strong> Example Company &copy; 2019
    <p><img src="[$FILE=1.png$]" />
</div> ]]>
        </dynamic-content>
    </dynamic-element>
</root>