The zoomviewer tool creates IIIF manifests with http (not https) image URLs.
For example:
https://tools.wmflabs.org/zoomviewer/proxy.php?iiif=Califate_750.jpg/info.json
{
"@context" : "http://iiif.io/api/image/2/context.json",
"@id" : "http://zoomviewer.toolforge.org/iipsrv.fcgi/?iiif=cache/1774f761854d805ec2e69fe35f683e63.tif",
"protocol" : "http://iiif.io/api/image",
"width" : 1492,
"height" : 1248,
"sizes" : [
{ "width" : 186, "height" : 156 },
{ "width" : 373, "height" : 312 },
{ "width" : 746, "height" : 624 }
],
"tiles" : [
{ "width" : 256, "height" : 256, "scaleFactors" : [ 1, 2, 4, 8 ] }
],
"profile" : [
"http://iiif.io/api/image/2/level1.json",
{ "formats" : [ "jpg" ],
"qualities" : [ "native","color","gray" ],
"supports" : ["regionByPct","sizeByForcedWh","sizeByWh","sizeAboveFull","rotationBy90s","mirroring","gray"] }
]
}The image URL here is http://zoomviewer.toolforge.org/iipsrv.fcgi/?iiif=cache/1774f761854d805ec2e69fe35f683e63.tif
Because the image URL is http, not https, it cannot be loaded from a secure context (e.g. an IIIF viewer served over https).