(German) How should the adjective in this sentence be suffixed?

[metaphyton]

Hello, will you help me to correct and figure out the language rule in this sentence below?

I wrote this today: "Ach ja, ein zarte Mann mit Harfe ist sehr nett ein Bild."

Then, thinking about the sentence, and the fact that "nett" is modifying the noun "Bild", I changed it to this:

" ... ein zarte Mann mit Harfe ist sehr nettes ein Bild."

Since the adjective is coming before the noun, does it take the -es ending?

May it even be: " ... ein zarte Mann mit Harfe ist sehr nett eines Bild"?

Can you help me to correct this sentence, and help me to understand the language rule at work?

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Also, as an aside inquiry: I am a man, but a partly feminine man. Somehow, I feel odd to express "Ach ja". Is this generally more of a man's expression, if speaking purely culturally about Germanic nations? In my head, I imagine it is to be uttered in a gruffer, manlier kind of voice, and cannot really hear it in my own voice, which is soft for a man, and still quite boyish even into my late 20s. I do not know if this matters, but it is kind of interesting to me.

[Louisa]

"Ach ja, ein zarter Mann mit einer Harfe ist ein sehr nettes Bild."