It's a good idea.
Few suggestions
1. There's no download button, it's better to put one.
2. Give a few different color backgrounds to choose from
3. By twitter banner you mean Twitter cover image? If so it is 3:1 aspect ratio, but the image you generated isn't following that, please check that
4. Resolution of the generated image is just too small, you can increase that
5. Some profile images just pop out, which is not a good UX, adjust that
6. Finally why have extra options to generate PNG, you could do that in the first attempt itself.
Thanks, have a nice day. Congrats on the launch.
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