Optimal image sizes

We have developed a backend on python/django/mysql and using django_photolog to manage all the uploaded images (6Gb).

I want to optimize this folder, to use it with Ionic App (lists with images (like blog posts list), view (blog post view) …) but is there any standard or recommended size documented somewhere?

I want to create a special folder, and store there images which are opmitimized for our ionic App. It means maybe to use A3NOME 500k image instead
of a 2Mb image…

Any help or recomendation will be apreciated