Well, as mysteriously as this problem started, it's now gone away again.

Traceroute shows much the same path and high latency on some of the hops, but images are loading successfully again.

Tracing route to image.rcuniverse.com [67.15.230.18]
over a maximum of 30 hops:

1 2 ms 3 ms 1 ms cpc2-warw5-0-0-cust517.brhm.cable.ntl.com [86.20.230.6]
2 9 ms 10 ms 8 ms 10.255.4.1
3 10 ms 9 ms 10 ms brhm-t2cam1-b-v113.inet.ntl.com [213.106.228.201]
4 19 ms 10 ms 9 ms brhm-t2core-b-ge-wan62.inet.ntl.com [213.106.231.189]
5 11 ms 12 ms 9 ms bir-bb-b-so-710-0.inet.ntl.com [62.253.185.149]
6 17 ms 11 ms 12 ms nth-bb-a-so-300-0.inet.ntl.com [62.253.185.105]
7 31 ms 30 ms 30 ms fran-ic-1-as0-0.inet.ntl.com [213.105.174.86]
8 32 ms 31 ms 33 ms po1-0.core01.fra03.atlas.cogentco.com [80.81.192.63]
9 28 ms 28 ms 34 ms ten13-0-0.core01.fra03.atlas.cogentco.com [130.117.1.221]
10 41 ms 45 ms 46 ms p15-0.core02.par02.atlas.cogentco.com [130.117.0.18]
11 43 ms 44 ms 39 ms p12-0.core01.par01.atlas.cogentco.com [130.117.1.230]
12 122 ms 122 ms 122 ms p14-0.core02.dca01.atlas.cogentco.com [66.28.4.206]
13 120 ms 121 ms 119 ms p15-0.core01.dca01.atlas.cogentco.com [66.28.4.21]
14 164 ms 163 ms 164 ms p3-0.core01.iah01.atlas.cogentco.com [66.28.4.90]
15 166 ms 161 ms 161 ms g0-1.na21.b015619-0.iah01.atlas.cogentco.com [66.28.64.62]
16 163 ms 163 ms 161 ms everyones_internet_2.demarc.cogentco.com [38.112.25.158]
17 142 ms 142 ms 145 ms gphou-66-98-241-28.ev1.net [66.98.241.28]
18 154 ms 153 ms 153 ms 66.98.241.105
19 157 ms 153 ms 153 ms image.rcuniverse.com [67.15.230.18]

Trace complete.