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 the use of it …
Edit: Mep. It's used in the remoteStorage app … Shouldn't it be there instead of core?

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