So why don't you just simulate the image uploading behavior for testing purposes (delay 500ms after clicking upload and then returning some pre-defined image URL)? I think that upload itself it's not important for you during development process.

My suggestion for the best API for testing and also production Unsplash is the best API. I don't know which language you are using but Unsplash provides a library for specific Languages so you easily used them.

API Doc: https://unsplash.com/documentation Specific Language Library: https://unsplash.com/documentation#libraries--sdks

I hope this API will you use.