We followed the guidelines here and used the management portal to add a sign-in page illustration and a banner logo image to our page. They work fine save for one oddity:
If you load the page and save the image to disk with Firefox or Chrome (let's just limit this to the sign-in page one for simplicity sake), it saves exactly as you know it to be, which inthis example is a 135KB JPG.
However, if you use IE to do the same, it's a 1810KB PNG.
Why?
This seems to be a universal problem with any customized page that I've tried, so it's not an issue with our image.
Yet, if you run the same test with Microsoft's default page (you may need to load the page in an InPrivate window), all three browsers download exactly the same thing....
This isn't a huge problem, but we do tend to use IE for these sites for compatibility reasons, and Microsoft's own instruction page tells us to keep the image sizes below a certain threshold. We are, but IE doesn't care.