SSL (Secure Socket Layer) is a technology to encrypt data transfer between computers. Sometimes your browser warns you about a missing or invalid certificate. These warnings can have three reasons:
We recommend to talk to the admin(s) of the affected friendica server. (Admins, please see the respective section of the admin manual.)
You can upload images from your computer using the editor. An overview of all uploaded images is listed at yourpage.com/photos/profilename. On that page, you can also upload images directly and choose if your contacts will receive a message about this upload.
Generally, you can attach any kind of file to a post. This is possible by using the "paper-clip"-symbol in the editor. These files will be linked to your post and can be downloaded by your contacts. But it's not possible to get a preview for these items. Because of this, this upload method is only recommended for office or zipped files. If you want to share content from Dropbox, Owncloud or any other filehoster, use the "link"-button (chain-symbol).
When you're adding URLs of other webpages with the "link"-button, Friendica tries to create a small preview. If this doesn't work, try to add the link by typing: [url=http://example.com]self-chosen name[/url].
You can also add video and audio files to posts. However, instead of a direct upload you have to use one of the following methods:
Add the video or audio link of a hoster (Youtube, Vimeo, Soundcloud and anyone else with oembed/opengraph-support). Videos will be shown with a preview image you can click on to start. SoundCloud directly inserts a player to your post.
If you have your own server, you can upload multimedia files via FTP and insert the URL.
Friendica uses HTML5 for embedding content. Therefore, the supported files are dependent on your browser and operating system. Some supported file types are WebM, MP4, MP3 and OGG. See Wikipedia for more of them (video, audio).