Thumbnail creation with Google Gemini appeared to be broken for a while. It wasn't giving me good results.
Today I tried using the latest version of Nano Banana and I got a great result. Here's the prompt and the resulting image.
"Design an eye-catching blog thumbnail for a post titled "How to Make Money With AI for Beginners." Include visual elements related to AI and money, such as robots, digital icons, currency, or futuristic graphics. Use bold, easy-to-read text and a clean, modern layout that attracts clicks."