Most embrodery designers are using underlay images when they digitize embroidery designs. It is important that if you later want to sell your digital embroidery designs, that you have not used underlay i mges with copyright. Old images without copyrights (because of the age), open source images/photos and AI images are some other sources. 

For the moment (march 2026) I can recommend to use AI image generation in paid accounts ChatGPT and Google Gemini for underlay images. With Google Gemini you can make embroidery designs from photos, with are quite amazing after my opinion.