Did Eru have physical form?

[Kuruharan]

Assuming he never actually took physical form, would not the people have imagined him as something?

I don't believe they thought about him that much. Most humans had no knowledge of him. The Elves knew that he existed, but they related to the Valar because the Elves were closer to them. Those humans who knew about Eru, also knew of the Valar and gained their knowledge from the Elves. The Edain tended to pick up the Elvish view of things so they too likely thought more of the Valar than Eru.

In terms of Arda, Eru was a mysterious figure that was entirely Other. Aside from the Valar (and even they had only an imperfect vision) the rest of the inhabitants of the world lacked the mental and spiritual tools to approach or understand Eru.

If they imagined him at all, they likely had some rather bizarre and inaccurate ideas of him.