A Microsoft file hosting and synchronization service.
“Accessed” includes software reading a file without displaying the picture. Your test of manually opening both images therefore doesn’t rule out other ways the files were accessed.
To investigate, check the original computer’s Security log for Event ID 4663, matching the filenames and time. It can identify the process and access type, but only if auditing was configured for those files beforehand and the logs still exist.
Without supporting records, the exact past action may not be recoverable. Matching timestamps alone cannot prove simultaneous viewing or a person’s intent.