Optimize pictures with optipng.

Just about 6 kiB of savings, so no huge improvement.

Just ran
```
function optALLthings() {
cd $1;
optipng -o9 *.png;
for dir in `ls -d */`;
do
	optALLthings $dir;
	cd ..;
done;
}
```
in core and settings (the only directories with pictures).

Wondering a bit what core/img/remoteStorage-big.png is about. grep -r remoteStorage-big doesn't help me in finding to use of it …