that sometimes happens when u load a custom character when you have no internet
Maybe use another image hosting site. I previously used FileGarden when it came to uploading the images to use in Party Project, but then I used Igmur [and now I understand how it worked to get the link for image*] the custom charaters's sprites are working for me.
It maybe time consuming but move the images from FileGarden to another Image hosting website, and try either copying the link/address of the image, and paste it to the .json file of which the image belongs to
*Igmur was the 1st image hosting site that I used for Party Project, but due to not understanding how to copy the image's address and instead copied the link, I switched to FileGarden to host the images instead