Teleposter got updated to 1.5.2 (rolling out via Google Play) with bug fixes and general dependency updates. Image upload should work again on newer Android versions. If you don't want to download it via Google Play, you can also find Teleposter on F-Droid (https://f-droid.org/packages/telegra.ph) or download the APK from the GitHub repository (https://github.com/jlelse/teleposter).