The API docs' example URLs can't be copied and pasted, as "%20" gets inserted. I'm probably not the target audience for the API, but it's a neat idea. I think people will end up with images described with tons of magic numbers that relate to each other in invisible ways and become unmodifiable and unmaintainable. Variables might help, as might relative positioning and sizing of elements.
Some feedback on the Designer:
The size setting dropdown is quite strange. Choices of unfamiliar destinations don't seem to make sense, and some of the things that do seem familiar come out an unexpected size and shape (e.g. "infographic"). The pixel sizes are clearer, but better would be handles on the canvas that can be dragged. There's also a typo: "Choose form a list of sizes".
Circles don't get resized? I can drag handles to make the apparent bounding box bigger, but the circle doesn't change: https://imgur.com/a/KhYluKj. Other shapes seem okay. Chrome 89 on Linux.
The tutorial walkthrough pops up every time I go to the designer, even though I've been right through it.
Seems like it would be a lot easier and a lot more powerful to use SVG instead of a giant string of URL parameters.
Your service could provide pre-made templates and an editor, and expose textfields, images, fonts, etc options via URL parameters. Then your service just has to render the SVG and return it as an image with the requested dimensions/format.
You can help by trying the API and giving honest feedback.
https://bruzu.com