By default, yiimp will adjust the difficulty of your miner automatically over time until you have from 5 to 15 submits per minute. It's a good trade off between bandwidth and accuracy.
You can also set a fixed custom difficulty using the password parameter. For example, if you want to set the difficulty to 64, you would use:
-o stratum+tcp://speedmining.net:3433 -u wallet_adress -p d=64
Here are the accepted values for the custom diff:
Scrypt, Scrypt-N and Neoscrypt: from 2 to 65536
X11, X13, X14 and X15: from 0.002 to 0.512
Lyra2: from 0.01 to 2048
If the difficulty is set higher than one of the a mined coins, it will be forced down to fit the lowest coin's difficulty.