Images are contained in the app
If the images are not downloaded they are contained in the app respectively it's APK file. You can download the APK file from your device (some file explorers allow to do so) or you can download it from an external source like ApkMirror, ApkPure, ...
Once you have downloaded the APK file you can simply unzip the APK file and then manually browser the directory structure or use your favorite file serach to to identify images and other useful files.
If you want to "dig deeper" (e.g. for extracting some string resources or other resources that are not directly accessible as a file within the APK file) you can decompile the APK using a tool like ApkTool (note Apktool is a Java tool and there requires an installed Java-runtime - JRE):
apktool d <path to the exported APK file>
You will get a directory containing thousands of files, containing the code, XML resources and images contained in the app.
Alternatively you can use a GUI program like Jadx (also needs Java, there is a version available that has an integrated JRE, hence does not require to download and install a Java run-time).
Using Jadx you can browse the path Resources/res/drawable*
to view and save a contained image of the app.
Images are downloaded
If the images are downloaded while the app is running the are usually stored in the private app-data directory in /data/data/<app packagename>
.
For more details on this topic see this question: Where Android apps store data?
To access this directory you need root permissions.
Searching for a specific file on-device especially if the file uses a wrong file extension or a different measure to hide it can be quite tedious. Therefore I would recommend to tar (tar is available on all Android devices by default) the whole directory, export it to a PC and there examine the content:
tar cvf /sdcard/app_data.tar /data/data/<app packagename>
Afterwards you can download the created tar file via adb (execute the command on your connected PC):
adb pull /sdcard/app_data.tar
As you are searching for images you may try to directly run photorec
on the tar file. photorec does not care about file extensions it directly operates on the data, if the image is in a known format and not obfuscated on data level it will be able to find and extract it.