fix(generateImage): broken generation png process fixed

Description

This PR intends to unbrake the png file generation

Fixes # (issue)

[ ] Patch fixing an issue (non-breaking change)
[ ] New functionality (non-breaking change)
[ ] Breaking change (patch or feature) that might cause side effects breaking part of the Software

Please describe the procedure to verify that the goal of the PR is matched. Provide clear instructions so that it can be correctly tested.

Any relevant details of the configuration to perform the test should be added.

[ ] I have followed the coding style guidelines provided by Centreon
[ ] I have commented my code, especially new classes, functions or any legacy code modified. (docblock)
[ ] I have commented my code, especially hard-to-understand areas of the PR.
[ ] I have rebased my development branch on the base branch (master, maintenance).