Open your calendar section from the menu bar on the left.
In the upper right corner, click on the + New Post button.
The creating post box will be displayed.
Then, at the bottom of the creating post box, you will see a few different icons. Click on the second one – the image one.
You can choose whether you want to drop the files, paste them, browse the device, or give a link to the file
Finally, confirm your choice and wait until the file will be uploaded.