Are the photos disappearing from the Gallery or Photos app, or are they disappearing from the Downloads app? If the latter, can you still see them if you use a file manager to look in /storage/sdcard0/Download? The Downloads app on some phones has an option to choose what time frame to show downloads from. If the default is something like "in the last week," then any file downloaded more than a week ago won't show up, unless you change it to "in the last 30 days" or "all."
EDIT: I just checked my wife's M8, and the Downloads app doesn't have that feature. Sorry.
06-04-2015 12:59 PM