Yes, they roll over until you've attained 12 months worth, in your case 120 images. Once you reach that level, you won't accrue any new image credits until you select downloads to drop you below the maximum of 120.
Rollover benefits are described on this page:
https://helpx.adobe.com/stock/faq.html
The 3-images/month plan, 10-images/month plan, and 40-images/month plan include rollover benefits, which ensure unused subscription licenses continue to be available in your account.
You can roll over 36-480 unused licenses month to month, depending on your plan, but if you cancel your subscription you'll lose any unused licenses.
Any unused rollover licenses cannot be used once your subscription has been canceled.
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