use image serialization format as defined in frontend REST API spec. refs #64

Images must be returned as object with alt property and multiple sources
for different screen sizes.
2 jobs for master in 3 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2083
test_buster

00:03:20

passed #2082
test_stretch

00:01:15