Pressing the TAB key (or Shift-Tab) moves the cursor between the question & answer fields. If you place the cursor into the answer field of the last card and press TAB you automatically get a new card.
Place the cursor on a card, then select the menu: Edit → Emoji & Symbols (or press Ctrl-Cmd-Space) to bring up the Characters window. Here you can choose between a number of different categories for all kinds of symbols and special characters. Double-click on a symbol to add it to the card.
If you use certain symbols very often, you can set up a special word or sequence of letters and have that automatically replaced with the actual symbol. Open the System Preferences → Keyboard → Text and add your sequence & symbol to the list. For example, add “−>” (minus + greater than) and have it replaced with “→” or “aaa” with the Greek “α”.
Press Cmd-Ctrl-Shift-4 to capture a selection on the screen as screenshot to the clipboard. Then click on a card and press Cmd-V to paste the screenshot onto the card. If the image is on a web page, you can also directly drag it from the page and drop it onto a card.
*Adding images works in the paid edition and the upgraded free edition.
Instead of using a mouse or touchpad to rate cards while studying, you can use keyboard shortcuts to rate cards as hard (Ctrl-1), unsure (Ctrl-2), or easy (Ctrl-3). Pressing Space reveals the answer and to navigate between cards without rating them you can use the arrow keys. For a complete list of keyboard shortcuts, open the app and select the menu: Help → Keyboard Shortcuts.