As I've looked at more established blogs, I've realized it's best to have photos that are essentially the same width as the text. In order to do this in blogger, it's easiest not to use those preset "small, medium, large" presets. The best thing to do is actually resize them using either Photoshop or PicMonkey.
To do this in Photoshop, open your picture, and press CTRL, ALT and I all at once.
This should open an "Image Size" box. Then, set the width to however many pixels across you require. My blog is best with 600 pixels, but experiment with yours. This works with horizontal and vertical photos.
Huzzah for looking a tad bit more professional!