You have to change the settings within that app itself. Not the phone's settings but the app you are trying to alter. So in the case of your tumblr app you would have to check the settings in the app to see if it allows for saving to the SD card rather than the phone's "Internal" storage. If it does not allow for that change within the apps settings, then you will have no choice but to save them to the "Internal" phone storage and then move them over to the SD card, which can be done in the phone's "File Manager" app.