You can't deliberately change them anymore (haven't for at least 6 years).
However, your start-up image will always be the splash page for the expansion the account logged in with the client has as its highest upgrade.
For example, many RtB accounts might log in to see AoS, SE or ML banners (not sure if all of them defaulted to ML when everyone given access to that content). I can't remember if the 7th Anniversary, Eighth Age and UO Gold had their own banner images - or if those images (and older ones) remain in the client.
One of my accounts logs in with the ML banner, and the other two to High Seas.