Can an iOS app that has access to Photos get all my photos?
Yes, if you give the application access to your photos and the application has network access, the photos can be taken from your iPhone.

Only use apps you trust


If you do not trust the developer, do not install or use their applications.
If the developer is not clear and explicit about how they handle your photos, do not install or use their applications.
If the developer is not contactable or does not answer your questions, do not install or use their applications.