The best way to add images to your events is to get the original event poster as a jpg file. They look great and stay true to the spirit of the event. But many events don’t have posters, so you need to look elsewhere. I recommend 2 sources: your own camera and Google search.
Even if you don’t have a great personal library of photos, a smartphone is capable of capturing high-quality web-ready images. Grab images from past-attendees, ask the organizer to to send you one of their’s, grab a photo of the venue, or take a close-up of something that could represent that event — a glass of beer to represent pub night.
If you don’t have time for any of that, try Google Image Search. Click on search tools and select Usage Rights and then “Labeled for Reuse”. Grab a higher resolution image and upload it to your event. Make sure to provide a photo credit and link to the original source.