Gmail now has an ugly overlay which obscuress any custom inbox image; this is a quick and dirty kludge for dealing with this $current_month Google UX mandatory decision (via developer console):
document.querySelector('.bkK>.nH').style.backgroundColor = "rgba(0,0,0,0)";
document.querySelector('.aAw.FgKVne').remove();
"transparent !important"; works for the backgroundColor, but not, for some reason, in a bookmarklet:
javascript:(function()%7Bdocument.querySelector('.bkK%3E.nH').style.backgroundColor%20%3D%20%22rgba(0%2C0%2C0%2C0)%22%3Bdocument.querySelector('.aAw.FgKVne').remove()%7D)()
(Generated with: Bookmarklet Creator with Script Includer)
Before: