You only have to cange a few configuration constants to disable ewikis support for uploading and caching images:

 #-- put this into your config.php
 define("EWIKI_SCRIPT_BINARY", false);
 define("EWIKI_ENGAGE_BINARY", false);
 define("EWIKI_CACHE_IMAGES", false);

If you're crazy you could even unset "*EWIKI_IDF_INTERNAL" (the "internal://" thing), what may however cause some side effects and problems).

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