Paving the way for the Voyager encounters, Pioneer 10 and 11 flew through the Jovian system in 1973 and 1974, respectively. They carried extensive particle and fields instruments, but due to the fact that they were spin stabilized, they did not carry cameras. However, one instrument, the Imaging Photopolarimeter, was capable of producing crude, 6-bit images in red and blue channels by scanning pixel by pixel, line by line as the spacecraft spun, slowly buiding an image. This worked pretty well for producing images of Jupiter's cloudtops, but the ability to image the moons of Jupiter was very limited. However, the Pioneers were able to produce the best images to date of these worlds, showing mottled albedo features that tantalized scientists. Pioneer 10 would have gotten a pretty good shot of Io, but unfortunately radiation effects prevented this. It did detect an ionosphere around Io, but this was detected through radio occultation. Pioneer 11 headed to Saturn, where it had a very successful encounter. However, when it reached Titan, a lot of data was lost due to the fact that Saturn was rapidly heading into solar conjunction. The images show the hemispheric brightness difference, the orange cloudtops, and little else (Titan alternately is brighter in its northern and southern hemispheres). It was a surprise that no other features in the clouds showed up, for we didn't know just how thick the clouds would prove.
Above are the best views of the Galileans and Titan. From left to right are Io, Europa, Ganymede, Callisto, and Titan. Some research was done using Pioneer images in the 1980s due to the fact that they had a red channel (the closest thing Voyager has is an orange filter). I would like to track down the original digital data for the Io image - if it were cleaned up, perhaps it could be used to look for change in albedo features compared to the Voyager 1979 images. This view shows it looking down on the north pole.
These views show these planet-sized moons on the edge of becoming worlds to us - these images made us wonder, but yielded few secrets.