I'm trying just now! Can I do it straight from my computer?
If the file size isn't too big, you can do it as an attachment to a comment. But if you took it with your iPad, I'm guessing the file size is big. You'll need to reduce it, either by resizing the image, or cropping it.
I've found a cheaty way to resize an image on my (now dead) Android phone is to open it in the gallery and take a screenshot of it.
Edited to add: But iOS devices should have better image editing capabilities built in. I'm sure you can resize it.