What is the difference between a standard image and a premium image?

• Premium images are from hand selected photographers and are expensive. See also here https://community.adobe.com/t5/stock/premium-vs-standard-content/m-p/10112005#M35253
• See here for asset cost: https://stock.adobe.com/plans
• The subscription allows you to acquire a limited set of asset types. So, you can't use subscription credits to acquire video, enhanced licences and premium images. Be sure you understand those limitations. Subscription credits roll over. So unused credits can be used the next month. This is good for up to a year's credits. With a 10-assets-a-month, you can accumulate up to 120 standard asset credits. Unused assets points expire if the subscription gets cancelled.
• You can use any of the free assets. If you take the stock subscription, you can use the 10 asset credits and then cancel. Be sure, however, to cancel in time.

If you need other assets like standard assets, you can either pay for the additional assets if you have a subscription, or you buy credit points, and you can use those against the assets. Unused credit points expire after a year.