[OIL-108] optimize image assets

[phogel]

...for release builds. Max file size currently at 409KiB for assets/images/background-large.png

[coveralls]

Pull Request Test Coverage Report for Build 234

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 24.393%

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Files with Coverage Reduction	New Missed Lines	%
dist/oil.bundle.js	23	24.39%
<!--	Total:	23		-->

Totals
Change from base Build 225:	-0.9%
Covered Lines:	872
Relevant Lines:	2891

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💛 - Coveralls

[Waschnick]

@subsend pls check if this is to your like & squash merge & delete branch if ready