Improve rendering of start UI.

The background image is hidden when a team which has a custom logo image,
allowing the logo to appear without any distracting and mostly occluded
image behind.

This also better supports PNG logos with transparency.
Clean up team logo images.

Java does a poor job of resizing images. This commit resizes all images
to have a maximum dimension of 250px, and where possible the background
has been removed so that the logo blends more cleanly in the application.