Selecting / deleting multiple or all picture files

Hello,

I have over 4,000 picture files to transfer to my newly acquired Meural Photo Frame MC315, and I cannot find a way to select multiple or all files to delete them, transfer them to the device, or move them into a library. All I can do, both on my.meural.com and the Meural Android app, is individually select the files one by one, which is obviously not an option due to the large amount of files to process.

Thank you.