... some questions for dummies

[Tim Curtis]

Yes, the thumbnails need to be regenerated when changing the resolution.

The Auto setting does the following, from the source code.

Code:

// Auto: Uses the device Pixel Ratio to set an optimum resolution and quality while maintaining the smallest file size (fastest image load time).
// NOTE: Manual should be used for Desktops
/*
Device        Physical    Pixel    Logical
            Res (px)     Ratio    Res (px)
---------------------------------------------------
iPhone 3g    320×480        1        320×480
iPhone 4s    640×960        2        320×480
iPhone 5    640×1136    2        320×568
iPhone 6    750×1334    2        375×667
iPad 2        768×1024    1        768×1024
iPad 3        1536×2048    2        768×1024
Samsung GS3    720×1280    2        360×640
Samsung GS4    1080×1920    3        360×640
iMac 27"    2560x1440    1        2560x1440
iMac 27" R    5120x2880     2        2560x1440
*/

if ($hires_thm == 'Auto') {
    if ($pixel_ratio == 2) {
        $thm_w = 200;
        $thm_q = 75;
    }
    elseif ($pixel_ratio >= 3) {
        $thm_w = 400;
        $thm_q = 50;
    }
    else {
        $thm_w = 100;
        $thm_q = 75;
    }
}
// Manual: Use the specified resolution and quality factor.
else {
    $hires_thm_wq = explode(',', $hires_thm);
    $thm_w = substr($hires_thm_wq[0], 0, 3); // The numeric part ex: "400" from "400px"
    $thm_q = $hires_thm_wq[1];
}