These two blocks of XQuery will search for page-image links in the header <biblScope> and in <pb> tags and report any that don't match the expected pattern. That doesn't mean they're bad, just that they need checking.

header <biblScope>s:

xquery version "1.0";

declare default element namespace "http://www.tei-c.org/ns/1.0";

for $b in //biblScope[@type="startPageImage"]
let $bits := tokenize($b/@facs, "/")
where not(starts-with($bits[2], $bits[1]))
or not(matches($b/@facs, '((co)|(rg7))_((g8c)|([0-9]{1,3}))_[0-9]{2,2}/((co)|(rg7))_((g8c)|([0-9]{1,3}))_[0-9]{2,2}_[0-9]{5,5}[rv].jpg'))
return (xs:string($b/ancestor::TEI/@xml:id), $b)

<pb> tags in the body:

xquery version "1.0";

declare default element namespace "http://www.tei-c.org/ns/1.0";

for $pb in //pb[@n]
let $bits := tokenize($pb/@n, "/")
where not(starts-with($bits[2], $bits[1])) 
or not(matches($pb/@n, '((co)|(rg7))_((g8c)|([0-9]{1,3}))_[0-9]{2,2}/((co)|(rg7))_((g8c)|([0-9]{1,3}))_[0-9]{2,2}_[0-9]{5,5}[rv].jpg'))
return (xs:string($pb/ancestor::TEI/@xml:id), $pb)