using "imagine"

hello,

I’ve checked Cambridge and Longman online dictionary and both of them provide usage of “imagine” with V-ing verbs

such as:

[+ -ing verb] She imagined herself sitting in her favourite armchair back home.

My question is, does the “sitting” in the sentence function as a adj. to modify “herself”?

If so, can I use a pp. instead, to suggest a passive voice?

for example:
I imagined my car fixed.
I imagined myself kissed by Lady Gaga.

I could’nt find any usages of “imagine” followed by a pp…
Is it ungrammatical?

Yes, that is fine; but there is usually another difference, in addition to passive-active: the past participle also generally denotes a finished action. In “I imagine myself kissed by Lady Gaga”, I am imagining myself in a situation where I still feel the imprint of her kiss on my cheek (or elsewhere), but she is probably already gone. To negate this finished aspect but keep the passive voice, use “being kissed”. For a finished but active action, use “having kissed”.