Note: if you're using someone else's images, make sure they're OK to reuse. You can use Google's advanced image search to search only for images that are free to reuse.
If you find an image that you like and that is free to use or share, be sure to click on View image. This will redirect you to where the image is stored.
There's no need to download the image. The image's URL can be used in your spreadsheet.
For each point in your map tour, you will need two image files: one at full size, and a thumbnail. Images should be in JPEG format.
According to Esri's instructions, "For full-size images, landscape (lengthwise) orientation is recommended instead of portrait orientation. The recommended image size is 1,000 pixels wide by 750 pixels tall (a 4:3 aspect ratio)."
"The thumbnail images should be in landscape orientation and ideally 200 pixels wide by 133 pixels tall (3:2 aspect ratio)."
You can resize your images using a graphics program such as Photoshop or Gimp. Gimp is free!
Your images will need to be publicly accessible on the web. You can do this in various ways:
Then you'll need to copy the links to these images into a column in your spreadsheet.