How reduce memory of gpu and system memory?

hi. i'm developing mmorpg with webgl 

it play on pc and mobile

mobile has shared memory with system and gpu

and memory is very low, specially iphone

our game are using png or jpg but it need very big gpu memory 

do you have any tips to reduce memory?

we are considering texture compressed but mobiles need different format

for example android etc1 or dxt1
                   ios pvr

is there tools to convert png to compressed texture automatically?

thx.. everybody