Try applying this as custom CSS:

.adminDrawerLogo img
{
   content: url(imagepath) !important;
}
imgLogoIcon 
{
   content: url(imagepath) !important;
}
.pageTitleWithLogo
{ 
   background-image: url(imagepath) !important;
}

https://old.reddit.com/r/jellyfin/comments/lc7ua6/how_do_i_change_the_logoiconname/

Let me know if you need the Jellico logo as any other sizes or something.