You have to first convert the image into a raster image as Roblox does not accept vector images at the moment. Once converted and if the converted image is in black color fill instead of white, invert the color space first. This is not required, but will be neat if you plan to change the color of the icon within Roblox.
Lastly, apply alpha bleeding to the image. You can visit this page if you are curious why this step is required before uploading. To apply, you can choose to use an image editor (e.g. GIMP), or by using an existing toolchain which is designed for this, such as: alpha-bleeder by AmaranthineCodices.
I used an automated process to upload all the icons for this plugin. You can check the shell script in the repository (for *nix only. Install WSL if Windows).