Daniel M. Ingram:
DISLEXICS of the Wold UNTIE!
If you want visual experiences, take a visual object. Try candle-flame meditation: look at the flame for a while, close your eyes, concentrate on what emerges where the flame was, when it fades or is gone, open your eyes, look at the flame, close your eyes, concentrate on what visually emerges, repeat again and again and again and again. Also, when waking up in the morning and going to sleep at night, pay attention to the colors on the back of your eyelids and play with those.
Second, concentration is not necessarily enough to get jhana. You must incline to jhana, incline to bliss, incline to silence, incline to rapture, incline to stuff that feels good, and resolutions for those things to arise helps, and taking any little bit of them as object and building it up to more than it started out being also helps.
Came here to say this. A candle flame or kasina object is best for visual imagery during jhana. I also was going to recommend watching the backs of the eyelids but it looks like Daniel beat me to that one too.

Also, keep a dream journal and do some
Active Imagination. These will really flex your inner eye muscles.