▼If posting via your browser:
1. Either click "Drop image" on the top page or click the "+" button and then "Upload"
2. You can add images by either pressing the "Click here to add images" button or "Drag and Drop to add images". Select as many images as you want to post and then click "Open".
3. Add in the required fields, like your comment and tags. You can also add more images.
4. Press "Upload" to finish.
▼If posting via the app:
If you enter image selection, you can add select many images. Pick the images you want to post, and then proceed to the next page.
Please note that if you can't post multiple images on Android, then it's possible your file selection app isn't compatible with selecting multiple files. For more information on apps compatible with multiple file selection on Android:
Try using this as a reference to try deleting your settings.