How to convert Image to BASE64 in webservice? Image size is 1mb

Hello,

1. In my webservice sciprt. I'd hit one mb payload. But Loadrunner giving me successful code. when I hit with 1 hits per sec but when i try to do the same for 150 hits per sec. All my transactions are getting failed. Only few passed. 

2. Moreover when i open the scipt in VuGen. It couldn't able to handle like "Vugent not responding". sometimes, "automically closed" .
LR version: 12.50