Brand-created Facebook posts that include images earn 87% of all engagement.
Tweets with images earn nearly 20% more clicks, nearly 90% more favorites, and 150% more retweets.
480 pixels wide is optimal for most social media.
Typically you don't have to write any text in a post, so the created picture can stand on its own, but if you want to make a clickable link you need to add that as text.
Avoid content longer than a normal screen, as e.g. Twitter will scale down such pictures, worst case making them unreadable.
If you insert an animated GIF it will become static, as the generated picture is always in PNG format. For the same reason, embedded videos will not play.
If you copy content from another site, it's likely that pictures won't show due to cross domain restrictions. To solve this, click the "Source code" button and copy/paste the picture URL to a new browser tab, save the picture and upload to Picit.
You can apply Bootstrap CSS classes to the content, to style it.