Question php --- access to /usr/bin (openbase_dir)

[MSplesk]

hello,

one of the websites i hosted on a plesk server (17.5.3) needs ghostscript.
server: ubuntu 14.04 lts; plesk: 17.5.3

the website was made in typo3. if i want to read vector-images (ai, pdf) i got the following error:
======================
Read pdf
Image generation failed
ImageMagick / GraphicsMagick handling is enabled, but the execute command returned an error. Please check your settings, especially ['GFX']['im_path'] and ['GFX']['im_path_lzw'] and ensure Ghostscript is installed on your server.

Read ai
Image generation failed
ImageMagick / GraphicsMagick handling is enabled, but the execute command returned an error. Please check your settings, especially ['GFX']['im_path'] and ['GFX']['im_path_lzw'] and ensure Ghostscript is installed on your server.
======================

maybe the openbase_dir directory isn´t set correctly. imagemagick and ghostscript was installed.
(/usr/bin/convert and /usr/bin/gs) i can change the openbase_dir setting to "none", but i think this
is a security risk?! how can i set access to this files/directory without security risk?

thanks for advices.