Can I use my own picture as a silhouette?

Yes. Under the pictures, "Your own pictures" has two buttons:

• "Draw with AI": type what the picture shows (for example "a sitting fox, a sailboat, a dinosaur") and your image engine draws it as a solid black silhouette. One picture on your OpenAI or Gemini key (the one Covers uses).
• "Add a picture": choose a PNG, JPEG, WebP or SVG of one dark shape on a light background, up to 15 MB. The outline is traced smooth and checked before a maze is poured into it. When it is accepted, the studio says "“Title” added: the mazes use it in turn with the others."