I am working on a website with a lot of pro photos. So I've decided to create 4 different formats for each photo:
*_sm.jpg - thumbnail
when you click on a photo you will get a bigger photo in the special gallery:
*.jpg - 1920 (width) for desktop version.
*_mobile.jpg - 1000 (width) for mobile website.
+ in the gallery there will be a link to the page with original photo 6000x4000.
By this method I have 4 formats and the user can open the source photo if he needs it.
Should I create a sitemap for images? Which of those have I to include there and which is better to block for index (in htaccess for example).