I’m a UI Designer on Roblox and am currently going through a phase of experimenting. I’ve been experimenting with different styles and just recently, I’ve started doing some icon/gfx design (I don’t really know the difference).
.PNG is usually a safe bet. .JPG tends to have compression that can become visible easily (so much so that it’s an entire meme within itself for a photo to have JPEG compression).
And I don’t tinker with SVG much so it might be better, but for me it’s been unnoticeable anyways, and more programs support .PNG
Yes, you would need a vector file to prevent such an issue.
PDF is the best example of a vector image, but I do not believe it is supported by Roblox Studio.