Remove error return from ExtractPictures

[paskal]

That function returns an error in a never expected condition, and that error would be logged message on the caller side: none of the callers handles it.

That change hides that error from the caller so that function would have a signature that better fit what it does and how it behaves.

[coveralls]

Pull Request Test Coverage Report for Build 848859630

• 6 of 8 (75.0%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.09%) to 83.646%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
backend/app/store/image/image.go	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
backend/app/store/image/image.go	1	87.37%

Totals
Change from base Build 848814691:	0.09%
Covered Lines:	5396
Relevant Lines:	6451

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💛 - Coveralls

[umputun]

makes sense