Comment to sharpest splash images
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80+ compression if it's a jpg/jpeg.
WebP is preferable, but still go 80+.
Depending on the type of image you may want to ensure that your upload at least @2 resolution when you upload the original. UNA will transcode to @1 for standard screens and @2 for hi-res. A raster image with @1 resolution will look bad on hi-res screen.