Thank you for your suggestion. We would love to show thumbnails to our mobile apps - unfortunately, this is a hard technical problem due to Boxcryptor's end-to-end encryption. As the photos are stored encrypted in the cloud, showing a thumbnail would require to download and decrypt the high-res photo. This is obviously not desirable on a mobile internet connection.
We have some ideas to solve this problem, but they cannot be easily done and thus thumbnails are not possible at the moment.