I’m not sure that this is possible – Redactor has some checks in its code to make sure everything lives within a block-level element like <p>
.
You might be better off using a Matrix field instead, with Text and Image block types. You can still use a Redactor field within your Text block type; just disable the Image toolbar button on it from your Redactor config in craft/config/redactor/. Give the Image block type an actual Assets field.
Then from your templates you’ll have full control over the image markup.
{% for block in entry.myMatrixField %}
{% if block.type == 'text' %}
{{ block.myRichTextField }}
{% elseif block.type == 'image' %}
{% set image = block.myAssetsField.first() %}
{% if image %}
<img src="{{ image.url }}" width="{{ image.width }}" height="{{ image.height }}" alt="{{ image.title }}">
{% endif %}
{% endif %}
{% endfor %}