New to this - what is image upload limit on gemini 2.5?

Depends where you're using it. The Gemini app and the API have different limits, and the app ones change with your plan, so any number someone quotes here could be out of date. The quickest check is to try a batch: it tells you straight away when you've gone over. If you're on the API, the limits are listed in Google's developer docs for that model.