Large table load slow mysql php high resources usage
check indexes and table structure

for 99% id - is INT
so 

SELECT picture_url FROM table WHERE id='1'


born scan of all column and convert it to archer, same if no indexes - it start scan all table and sort

at the same time 

SELECT * FROM table


just start send data without transformation

if I right - just change query for 

SELECT picture_url FROM table WHERE id=1


and add index if it not already there