I am not sure but I get the following error
Unknown column 'tsk.taskid' in 'on clause'
Here is the SQL statement
SELECT *,count(requestID) as requestCount FROM tasks tsk,tasks_categories,tasks_gallery,measure,auth_user left outer join task_user_request tskr on tsk.taskid=tskr.task_id where measure_id=measureid and userID=id and catid=cat_id and photoid=photo_id AND tsk.taskid=tskr.task_id GROUP BY userID,url
I suggest you make the whole statement ANSI format to avoid confusion and problems (like you also having tsk.taskid=tskr.task_id in the where clause which will invalidate the outer join).
So use ANSI for all joins ... you will need to fix the ON clauses
SELECT *,count(requestID) as requestCount
FROM tasks tsk
JOIN tasks_categories ON ??
JOIN tasks_gallery ON ??
JOIN measure ON ??
JOIN auth_user ON ??
left outer join task_user_request tskr on tsk.taskid=tskr.task_id
GROUP BY userID,url