Decal uploader live for all but my logos didn't work

Yes you can.

Use Adobe Photoshop CC.... HOWEVER, the svg feature in Photoshop CC is not compatible with what GT Sport uses, so you will have to edit the images to your liking in Photoshop and upload that image to a .PSD to .SVG converter.

After getting the file size under 15KB, I converted and uploaded the image, but all I saw was a black silhouettes of where my image used to be.

Your problem is ".PSD to .SVG converter" they're working horribly for this.
 
Some logos you download have licensing (mainly ones you get from Wiki) info embedded that prevent them from being seen inside GTS. You get a blank box. There are way around it if you look through the code or use illustrator/corel draw to redraw it. Another options is check for the logo on seeklogo.com or brandsoftheworld.com. Also a huge reminder. DO NOT USE compound paths. They will jumble the logos.
 
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