compute image: typo & clarifying future role of "default"
compute image: typo & clarifying future role of "default"

sagemathinc/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc