convert input.jpg -rotate 90 output.jpg

Also can be used to change from format to another

convert input.png output.jpg

For image captions, I settled on a beautifully simple solution posted by Andrew Wei on Stack Overflow:

 image_caption

This takes advantage of the fact that you can create CSS for combinations of HTML elements. In this case, I can use

img + em { display: block; text-align: center;}

To target the image_caption text only and center it under images, which are also centered on this site by default. It works perfectly, and this isn't even Jekyll-specific. Anyone publishing in Markdown could do this.