I'm also having problems with mintUpdate!
The startup one
doesn't show an icon, but mintUpdate on its own or from the menu does.
Perhaps it has something to do with the icons directory?
When I try to change the icons from preferences it automatically goes to /usr/lib/linuxmint/mintUpdate/tmp folder, but they're actually in the /usr/lib/linuxmint/mintUpdate/icons folder?
Also, another bug I see is "User will login in ** seconds" still shows in the login screen even after I unchecked Enable Timed Login.
I tried it with different values and it still shows up when unchecked, doesn't count down, just stays there.