It's from Old French all right, but probably not because there was no existing word. Old English Translator[1] suggests geþanc (a bit similar to the modern German Gedanke) or the verb wénan, both of which can be confirmed by Bosworth & Toller's Anglo-Saxon Dictionary[2].

The Normans introduced French as the prestige language of England when they invaded, but Anglo-Saxons would have wanted to express the concept of imagination way earlier. If they had wanted to, they could have borrowed the word from Classical Latin imāginor without the 'help' of the Normans.

[1]: https://www.oldenglishtranslator.co.uk/index.htm

[2]: https://bosworthtoller.com/

Imagination is an English word, because it obviously is. Common man's definition. The sky is blue, I don't need to prove it or define blue.

My bacon came from a pig. It doesn't mean the bacon is a pig. It's breakfast. An English breakfast. And the French are pigs. See?