Why does it say the deleter is unknown?

Either the bot is missing View Audit Log, or the person deleted their own message. Discord keeps no record of that, so nobody can know.

Can it recover a deleted picture?

Yes, with the media log switched on. Coverage starts from when you switch it on, and files over 8MB aren't kept.

Newt keeps a record of what happens in your server: deleted and edited messages, joins and leaves, bans, nickname and role changes, voice, and more.

One command builds the log channels for you. The media log even keeps the file when a picture or video is deleted.

The quickest way is /logging setup. It builds a hidden category of log channels for you, points every event at the right one and switches the lot on. Pick a few channels grouped by type, one channel for everything, or a separate channel for every event. Run it twice and it reuses what is already there rather than making a second set.
If you would rather do it yourself, run /logging channel and pick where the log goes. Everything is switched on at once, so that is the whole setup for most servers.
If you want a particular kind of log kept apart, open the dashboard, find Server log, and give that one its own channel. Anything left on Same as above keeps going to the main channel.
From Discord, /logging event does the same thing one at a time, and /logging status shows you what is being recorded and where.

Command	What it does	Who can use it
`/logging setup` [style] [category]	Build the log channels and switch everything on in one go. The category is hidden from everybody but staff.	Manage Server
`/logging channel` [channel]	Send every kind of log here, and switch them all on. Leave the channel out to turn logging off.	Manage Server
`/logging event` <event> <on> [channel]	Switch one kind on or off, or send it somewhere of its own.	Manage Server
`/logging moderation` [channel]	Where numbered moderation cases go. Leave the channel out to switch it off.	Manage Server
`/logging media` [channel]	Where deleted images, videos and voice memos are kept, with the file. Leave the channel out to switch it off.	Manage Server
`/logging reminders` [channel]	Where survey reminders are recorded. Leave the channel out to switch it off.	Manage Server
`/logging pause` <duration>	Mute the server and media logs for a while, so a raid cleanup doesn't flood them. Capped at 6 hours; they come back by themselves.	Manage Server
`/logging resume`	End a log pause early.	Manage Server
`/logging status`	All four logs at once: what each records and where it goes.	Manage Server

Is it free?: Yes. Every log is free.
Why does it say the deleter is unknown?: Either the bot is missing View Audit Log, or the person deleted their own message. Discord keeps no record of that, so nobody can know.
Can it recover a deleted picture?: Yes, with the media log switched on. Coverage starts from when you switch it on, and files over 8MB aren't kept.

A button panel that opens private support channels, with transcripts.

Ban, kick, timeout and warn, with every action kept as a numbered case.

Rules that deal with spam, bad words, links and raids without a moderator.

Greet new members with a message and a picture card, and say goodbye.

Let members pick their own roles with buttons or emoji reactions.

Give every new member a role the moment they join.

Free, and nothing runs until you switch it on.