In this example we will apply the content from a module headline to the alt text of an editable image.
To start off, we'll make an editable headline
<editable name="headline">this is the headline</editable>
Looking at the Implied Default for Editable Text, we can see that the field is named "content">. We can then take the content from this field and apply it to an <img> tag, using the replace attribute:
<img src="http://placekitten.com/200/300" width="200" height="300" replace-alt="{{editables.headline.content}}" style="display:block">
If we then want to make the image editable, we need to open it's editable fields, which can be seen in Implied Default for Linked Image:
<editable name="" label="">
<field type="src" name="src" label="Image Source"></field>
<field type="number" name="width" label="Width"></field>
<field type="number" name="height" label="Height"></field>
<field type="text" name="alt" label="Alt text"></field>
<field type="href" name="href" label="Link URL"></field>
<a replace-href="{{href}}" rule="{%remove_outer_unless href%}"><img replace-src="{{src}}" replace-width="{{width}}" replace-height="{{height}}" replace-alt="{{alt}}"></a>
</editable>
Because we are taking the alt text from elsewhere, we can remove the alt field from the editable image, and change the implied replace-alt so that it references the content from the headline editable instead.
<editable name="headline">this is the headline</editable>
<editable name="image">
<field type="src" name="src" label="Image Source"></field>
<field type="number" name="width" label="Width"></field>
<field type="number" name="height" label="Height"></field>
<field type="href" name="href" label="Link URL"></field>
<a replace-href="{{href}}" rule="{%remove_outer_unless href%}"><img replace-src="{{src}}" replace-width="{{width}}" replace-height="{{height}}" replace-alt="{{editables.headline.content}}" style="display:block"></a>
</editable>