Yeah, I think it's time for a refreshed image of the "amusement park skyride" that's been on the homepage for at least six months now.
Out of curiosity, I did a little research and this is actually pretty easy to do with browser extensions, for example Firefox has one called Stylish that lets you write your own CSS rules:
I believe Chrome has something similar.
Using your browser's element inspector to figure out the ID or class of what needs to be changed, you can edit the style of pretty much anything you want. Keep in mind that if you're seeing some UI performance problems in Marketo, after clearing your cookies and cache, you'll want to try disabling extensions like this one as well.
Here I use the element inspector to find the ID of the div where the background image is set:
Here, I create a rule in Stylish to override that element's background-image property:
And here's the result in My Marketo: the background image that loads is of the moon Pandora from Avatar:
Have fun, and remember to keep it SFW! But keep in mind that if you do this, you'll miss out on the new images we set to be the default background