Here’s another method that doesn’t require a third-party app. It also can be performed solely within the Discord app. It’s done by creating a workflow in the Shortcuts app. There are two workflows to choose from. One gets the last screenshot, converts it to a JPEG and then prompts you to share. The second one prompts you to select a photo or photos from your photo library, converts it or them to a JPEG and then prompts you to share. You can change the JPEG quality by opening the shortcuts in the Shortcuts app and adjusting the slider in the Convert Image action.
Download the Shortcuts app from the App Store if you don’t have it installed. You’ll need to enable the Shortcuts button in other apps and enable the shortcuts to run in other apps. Instructions on this page of the Shortcuts User Guide.
Here are the download links to download the shortcuts into the Shortcuts app from Safari on your iPad:
Share My Screenshot

Share My Photo

The sequence of steps for the workflows from within the Discord app:
Share My Screenshot
1. Tap an existing photo in the text channel. 2. Tap the photo. 3. Tap the share button. 4. Tap the Shortcuts icon. 5. Tap Share My Screenshot. 6. Tap the Discord icon. 7. Select a server, select a text channel, add an optional comment and tap Post.

Share My Photo
1. Tap an existing photo in the text channel. 2. Tap the photo. 3. Tap the share button. 4. Tap the Shortcuts icon. 5. Tap Share My Photo. 6. Select a photo or photos. 7. Tap the Discord icon. 8. Select a server, select a text channel, add an optional comment and tap Post.
