In the Tumblr Editor, open your Account Settings and select Edit Appearance.
On the next page, select Edit Theme.
Click on the Edit HTML link.
Paste this exact script into your Tumblr HTML file. It's best to add it between the Head tags. Update, then save.
Create Text Post
Return to your Tumblr Admin page, and create a new Text post.
Paste this shortcode into the post and save. Expand the post to see your new Image Resizer plugin. Click the POWr Settings Icon to set up POWr Image Resizer.
Edit your Image Resizer on Tumblr:
Go to your live Tumblr website and click the gear icon in the upper right corner of your Image Resizer Plugin.