Accessing Photos from other Apps
Say, for example, you've imported an image from a camera or other source. You can see the image in the Android Photos app. When you go into Instagram though and try to open the image, Instagram defaults to the 'Gallery' view. You then have a drop down for selecting other folders, including 'Other' which then gives you access to all photos. Not terrible, but not so simple. Other apps can be worse than Instagram, making only the Gallery images available.
The more reliable solution is to start from the Photos app itself. Find the image you want to use in another app, then use the 'Share' button (bottom left) to import that image to the other app. That should work for almost any photo app that you want to use.