The editor is Atlassian's own, the same one you use on a work item. Headings,
bold, lists, quotes, code blocks, text colour and links all work, and typing
/ opens the quick insert menu.
Paste an image address into the field below the editor and press Add image. The picture appears underneath the editor straight away. Check that it shows before you save. If it stays blank, the address is wrong or the image is not reachable without signing in, and everyone on the dashboard will see a broken picture instead.
The address has to start with http:// or https://, and
the image has to be readable by the people who look at the dashboard. A gadget
holds up to 20 images.
Open the gadget's menu (the three dots in its corner) and choose Edit.
In the gadget's own configuration on your Jira site. There is no server and the app asks for no Jira permissions, so nothing leaves your site. Deleting the gadget deletes the text with it.
The gadget says so at the top when less was stored than was saved. Open Edit, shorten the text or remove an image, and save again.