a) No images just descriptive narrative that the ships' communication array is picking up a weird repeating signal on an archaic communication band, language or code unknown, but when the PC's finally crack the code they realize it is in "radioglyphs". I suggest using the Dah-di-dit type formula in Morse Code if you want to describe what they are hearing or go with repetitive shapes such as square, triangles etc for a visual angle.

b) You could have several prepared pictures, some that are gibberish and some that clearly form images and have structure... I would make the structured pictures first on graph paper, and then do the gibberish pages by just mixing up the same number of graph blocks on a second page. 

c) You could use an existing code or make up your own based on an existing code that you can get info on... the easiest in my opinion to mess with would be Morse or one of it's related foreign codes. You could use the Chinese or Japanese version. 

Anyhow depending on how the PCs do and how you want to use the hook they can get curious and follow the transmission to the adventure without really decoding it or successfully decode portions or all of the message or totally mistranslate something important.

How you use the radioglyphs would be up to you the Ref as far as what race is sending/sent them, or for example are the glyphs part of an archaic navigation system that if decoded opens up to the PCs a whole new (hopefully safe) Jump Way or Ways, or is it a hello, a plea for help, a warning to stay away, the history of a dead race, a SOS of a ship and so on.

So what is a radioglyph? As best I can gather in it's most basic form is a series of short pulses, long pulses and silent pauses that convey math, vocabulary and syntax (first purposed language Astraglossa by Hogben... which is suppose to be based on the Mayan Calendar but seems to me to be Space Morse Code.) In more complicated forms such as Philip Morrison suggested used shapes such as squares and triangles (not a bad idea). In addition a second language is said to have formed called Lincos out of Astraglossa (I have done no in depth research on this language yet or Philip's shape based system.) Keep in mind I am no expert on space transmissions this is my understanding of what few lectures I have read and articles I have found on the net. I am attempting to actually understand Astraglossa based on what I have been able to find on it but I think there are transcription errors in the formulas. Most of the latter ideas that seem to have been used or suggested seem to involve an "easy" math clue that allows the receiver to figure out how to decode the message and get a actual picture (thus the suggested graph paper idea above as many of these space messages finnish with a sort of bit or dot matrix image). The clues have turned out not to be easy, one scientist gave his human peers the message they failed to decode it so I do not think an alien would figure it out either. (I think we actually sent that message into to space if I remember what I read correctly). Apparently the folks coming up with the codes are not following the KISS rule well. Personally I would give the lessons to people without Phds to decode to see if it was indeed simple enough. If a regular person can solve the puzzle then maybe E.T. has I better chance. 

Of interest for Sci-Fi buffs Hogben seems to suggest an Entrance Examination as part of the first lessons... a reoccurring theme in Sci-Fi from aliens to us. He also suggested that the exercise in trying to figure how to communicate with an unknown species and teach them a language, might give us insight into how they would try to do the same. 

My Thoughts on the subject of Astroglossa:

I personally find devising a message that is intended not to be simple as part of your attempt to communicate and teach a language indicates an insincere desire on the part of the sender to be understood. Hogben also seems to jump steps in the learning process that might appear unnecessary to him but are in fact absolutely necessary for teaching and learning.

Hogben also appears to despise people who educate children thus ignores any research on that, felt how adults learn a new language irrelevant, and he made no attempt to consider how other species communicate or inter-species communication occurs and how humans train/teach other species (dogs, horses, birds, seals, dolphins etc all came to my mind). I do not know if other researchers have continued in this vain of thought that pure math with limited learning process is best.  

Seeing how Morse Code is in basic principle similiar to Astraglossa I find it odd there was no mention of it in his work. I investigated how Morse is and was used and for all intents and purposes it has radioglyphs of the sort purposed by Hogben and I am sure a code could be devised to use it to create images as well as purposed by others. Morse further has the advantage of being experiemented with in use for a long period of time. 

Just some thoughts and maybe a fun idea to use in the game.