Basically, I was trying to submit a custom decor, however when I put the links in the correct boxes, it either says ‘Couldn't open item image.’ Or ‘item image not valid’

I’ve read loads of guides, I tried using imgbb and imgur, my sizes are correct (100x100 for thumbnail, 460x500 for decor), I used the image address links and .png on the end of the links, and the images themselves are pngs. I’m so confused