Then click the Image Selector button:
Then click All Images (Click for additional options) and then choose Upload New Image:
Then click the Choose button:
Choose your file, and then click Upload:
*Please note, you can link to PDF in a "Long Description" field of an Event, but not upload one to display in the "Long Description". If you are able to make the PDF an image (.png or .jpeg), then you would be able to upload it as an image in the "Long Description" field.
Resolution: Go to the event that you are wanting to upload your image to, click on General Settings under event settings:
Then scroll down to the Long Description and click the Magnifying Glass: