Fix tour image sizes on non firefox browsers (#3171)
Fix tour image sizes on non firefox browsers (#3171)

- fixes #3170
fzzzy/testpilot,lmorchard/testpilot,mozilla/idea-town-server,fzzzy/testpilot,fzzzy/testpilot,flodolo/testpilot,chuckharmston/testpilot,lmorchard/idea-town,chuckharmston/testpilot,lmorchard/idea-town-server,lmorchard/idea-town,lmorchard/idea-town-server,meandavejustice/testpilot,flodolo/testpilot,meandavejustice/testpilot,mozilla/idea-town,lmorchard/testpilot,lmorchard/testpilot,lmorchard/idea-town,lmorchard/idea-town-server,lmorchard/idea-town-server,mozilla/idea-town,mozilla/idea-town-server,meandavejustice/testpilot,mozilla/idea-town,chuckharmston/testpilot,lmorchard/testpilot,mozilla/idea-town-server,mozilla/idea-town-server,flodolo/testpilot,mozilla/idea-town,flodolo/testpilot,chuckharmston/testpilot,lmorchard/idea-town,fzzzy/testpilot,meandavejustice/testpilot