The shareable link that you get from Google Photos cannot be used in many applications, it doesn't just show the picture, it also puts it in a display showing the date (and any other pictures that you have included in the same shareable link)

Instead, you need to:

• In Google Photos, click into the picture you want to use, so it is the only one on the screen
• Right click, and choose Copy Image URL (that's the option in Chrome, other browsers have slightly different words for the same action)
• Paste that Image URL (not the shareable link) into the Image-URL field in Maps.

While on Google Photos click Upload, (probably upper right corner), then click "Google Drive". If for some reason that didn't work, just read this short official Google help made to answer this exact question, it will tell you what's wrong.

If after clicking upload "google drive" doesn't show up, It's probably because your account is work/school. One workaround is to share the photos to a non-educational account, and follow the method given at the beginning of this answer.