When I put the path to ImageMagick and test it (/usr/bin), seems to find it (path correct), but a general error occurs :
Omeka has encountered an error
To learn how to see more detailed information about this error, see the Omeka Codex page on retrieving error messages.
I did both solution to catch an error (log files and .htaccess uncomment "development environment" like the codex page says.
-> no log appears and no other error message.
I've no way to go this problem further. Please help me !