On an iPhone, once you have selected a photo or photos, click Options, usually in lower left corner.
This lets you select a photo file size. I usually choose medium. Some are too large to upload.
Then hit Add, upper right.
Wait for the photo to show up in the post.
Now tap on the choice in tiny print on the thumbnail size to select full size or thumbnail.
Before you start the process, set your cursor where you want the photo top edge to be.