It looks like that apk to jpeg conversion is something which is not commonly done. It relates to attempts to convert Android app (.apk) to JPEG picture (.jpg, .jpeg), something that is not typically possible.
However, sometimes .apk packages from games or other apps can contain pictures that some users want. If that's your case, you can simply extract the content because APKs are essentially ZIP files.
Note: Some apps produce screenshots, but these are not saved inside the programs .apk file, but in the generic picture folder.Open APK file Open JPEG file