It's even easier than that. Select it as "caption this template". Then just right-click on the image and Save As. It will save as a .png. Then upload what you just saved.
In fact, you can save any variation by right-clicking. You can select an image, then use Add Image to add more images, you can add text, etc. Then right-click and save. It will save everything.
For instance, you might want to change text colors or have multiple versions. Set the font color, save, change the font color, save again (different name), etc.