For a long time, users couldn't easily expand profile photos. Recently, functionality has been improved across different platforms:
Because Threads is built on the Instagram infrastructure, you can often view the same user's profile picture on Instagram by tapping it (or tapping and holding if they have an active story) to see the high-resolution original. Common Fixes for Profile Picture Not Showing threads profile picture viewer fixed
Many "not showing" bugs are resolved in newer software versions. Check the Apple App Store or Google Play Store for the latest Threads update. For a long time, users couldn't easily expand profile photos
Threads Profile Picture Viewer Fixed: Complete Solution Guide Check the Apple App Store or Google Play
If you are using the web version, certain security extensions like Bitdefender Anti-tracker have been known to block images on Threads. Try disabling your extensions one by one or adding threads.net to your exceptions list.