Is there a limit to the number of photos I can load into an app?

There is no limit of photos per app.  Each gallery section in an app has a limit of 150 photos, but you can have multiple galleries in the regular or professional version.  

Here are some considerations:

Various mobile devices have a limit of the maximum size they can be in order to cache the images and view the app offline.

The more images you app has, the longer it will take to load. 

So while there is not a technical limit, there is a practical limit.   

Experiment with various app sizes and see which size is optimal for your usage.