Your question consists of multiple parts.
Let me try to answer them, one by one.

What can I do with Adobe XD?
Is it just for prototyping and designing for apps?

Yes, and websites, and even devices with a voice interface.

Except for logo design, can I photoshop images or create new images?

Adobe XD nor Photoshop are appropriate tools to design logos, and you can't edit photographic images in Adobe XD. But you can create elements like icons, symbols, diagrams, and simple illustrations in Adobe XD.

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