Well, the thing with roblox endpoints is you can’t actually get them from in studio, you would need to use a proxy. This pretty much means that studio would contact your server, and your server would contact the endpoint. Explaining how to set up one is a bit out of scope of this question, but just know there are many good explanations of it. Also, know that it involves either setting up a server with port forwarding or purchasing a cloud server. A good and simple explanation of the code side of the proxy can be found here. Keep in mind that tutorial is assuming you’re using node.js for the proxy.
Just a note, your command to access the api is also incorrect, this is how it is supposed to look, https://inventory.roblox.com/v2/users/{UserId}/inventory/13?limit=100. Note that the highest amount you can get is 100 decals at a time.
Edit: After further reading your question, I just wanted to say getting images from a user doesn’t really work, and the command I’ve given used 13 which is decals which should probably work for your needs.