This is pretty fascinating stuff.
For me, I mostly have an inner monologue, although I can sometimes purposefully or accidentally change the sound, tone, or gender of the voice in my head, i.e when I'm reading a book, trying to write a scene for some characters, or just by random while thinking about whatever (hence accidentally). I've been able to make my thoughts take on certain accents, like a New Yorker accent or a Russian one, but it doesn't usually last very long and I don't have very fine control over it. It usually only happens when I read one of those posts where it's like, "(in x voice) [insert funny statement here]" or something like that. And even then, if I don't know what type of voice they're talking about, it just defaults to the generic genderless and featureless voice I have in my head.
I can picture images as well, but like Hellothere said it's just in flashes. Sometimes they can be detailed, but only if I have a really good grasp of what the object/person/thing I'm imagining looks like, like with an original character, and even then things usually get fuzzy in places and at the edges. If I focus I can get a little bit more than a flash, or a rapid series of those flashes of various scenes or images in my head, but not much more than that.
Sometimes I get what I guess you could call "blank periods" in my head as well, where I'm not outwardly thinking in monologue and when I realize it I start thinking in monologue again. Since I'm usually not focusing on my thoughts during those times I'm not usually aware of what exactly I think about specifically, although I think it often happens during my job when there's a brief gap in orders I have to make or during a slow period and I'm not doing anything but waiting. In those times I think I just get a general sense of "I'm bored" or "When will the next order happen/what will it be" without really articulating it in my head with words or images. Not really super sure on that, but I think it happens.
I'm also able to keep a full conversation with myself in my head to keep myself from getting bored when I have a lot of downtime during my job or when I'm on autopilot, and I narrate stuff that I read/write in my head as I'm writing it as well. Like right now, I'm thinking the words as I type them, the words kind of stretched out/jittery as my fingers move across my iPhone's keyboard. I don't know how else to describe it but that, but hopefully I manage to get the image across.
Edit: also, I don't think the voice my inner monologue is usually in is my own voice. But it's still me. It's just sort of generic and genderless to me without any strong defining details. Is it like that for anyone else?