Ok, I finally figured out how to add an image to reblogged photo posts. If you’ve noticed, new tumblr doesn’t let you do this, it only lets you add pictures that are already uploaded somewhere else and that linked image is always a shitty grey box.
1. Click the reblog button on the post you wanna reblog and start to make your rebloggy post.
2. Then, in a new tab or window open tumblr and start to make a text post.
3. Click the "Upload Image" button and upload your image
4. Right-click the image and "Copy Image URL". Now you’ve got a tumblr-uploaded URL!
5. Go back to the post you’re in the middle of reblogging from Step 1 and click the "Insert/Edit Image" button.
6. Paste that into the “Image URL” field.
and there you go!