Why are my jpeg files 0 bytes, and how can I open them?

If I download jpegs to one coimputer toi my dropbox folder there, the files show up with the appropriate name on the dropbox folder in another linked computer, but they show up with 0 bytes. If I copy the jpegs and paste them somewhere elsewhere, the jpegs then miraculously change from 0 bytes to the correct size; if I duplicate the files in the folder, the same thing happens; the duplicated file, and the original, now both no longer show 0 bytes but the appropriate size. I obviously cannot open the files when they show 0 byte size, but I can open them normally after they have been copies or duplicated.