Open up an issue and tap on the image icon to the right of the photo preview area.
On the pop-up menu that appears, tap on "Import File From...".
This will open up a new screen showing the other sources you can import an image from.
The first time you go through this process, it may show your device files. Tap "Browse" in the top left to show the other options available - these will be based on what other apps you have installed on your device. From that point, it's simply a case of navigating to the location of the image you wish to add.
If you do not see your app listed when you're on the screen above, tap the three dots in the top right and further options will be displayed, with toggle switches which can allow you to enable these options to be displayed as default in the future. You'll also be able to disable other options, or re-order this list if you wish. If your app of choice is still not listed at this stage, then it is not possible to import photos from that app. Unfortunately, this is out of our control.