Manual Resizing You don't have to use HTML to resize a picture in your Tumblr post. Small handles appear around an image in the editing box when you click it.

Double-click the image file you'd like to include in your post. The File Upload window closes and your image appears in the editing box. Click the "HTML" button and review the HTML code that appears.

The action you use for this is Scale Images. Drag it into your workflow and you see that you can configure the pop-up menu to allow you to scale the image to a specific pixel width—640, for ...