If you mean the account banner, that's just "Banner URL" in profile settings.

If you mean images in posts, Sparkler uses Markdown and HTML syntax. The Markdown for images is ![alt text](url), you can search "markdown cheat sheet" for more.

If you mean background images, you can use url(https://example.com/image) in any field where you would enter a background color. But you should use these sparingly.