You can't upload images in a private conversation and then use them in your sig or in a thread because those files are private only to the users in the conversation. However, the attachments above should work fine. So update all the images in your sig to the files which start with "4874"
Old names are freed up after not being used for three years. Themes are fair game, but if anyone tries to do anything sneaky, that's also what the mod approval layer is there for.
Since iOS doesn't support OGG files natively, it looks like you will need to download an app such as VLC and paste the URL to the OGG file under Open Network Stream.