Here are some common problems I am finding:
This is usually because your laptop is more forgiving on mixed case with filenames: a link named photo.html that refers to a file named PHOTO.html might work on the computer, but not on the Opal server or other servers you might use in the future.
my photo.jpg is not best practice. Use an underscore or hyphen: my_photo.jpg or my-photo.jpg or even myPhoto.jpg. Capitals are okay as long as long as you remember that myphoto.jpg and myPhoto.jpg will not work on Opal if your link and the image are not exactly the same. They must be exactly the same.
Use a CSS class. Put the CSS in your stylesheet and class="class-name" in your html. This is a CSS validation that is your responsibilty, not a theme problem.
img.align-center {
display: block;
margin: 0px auto;}
<img src="photo.jpg" alt="Magnolia" class="align-center" />
Don't forget your alt info either.
https://www.w3schools.com/cssref/tryit.asp?filename=trycss_text-align
is not best practice for web design. git rid of the spaces.
it's not my job to check what you have flat out missed!! open the grade sheet and check to make sure you are doing all the required things. If you have a question or it breaks, that is okay. I will help with that. Refer to the 2.2 task page on the web site.
make sure you only have only one head section and one body section.
if you are using the 2.1 theme you make your clicked page active like this: <a class="active" href....