diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index fd2949492f6..2209cb98d5d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -77,6 +77,8 @@ jobs:
         run: scripts/test-release-ref-contract.sh
       - name: Relay image eligibility contract
         run: scripts/test-relay-image-eligibility-workflow.sh
+      - name: PR image URL event contract
+        run: scripts/test-pr-image-url-event.sh
       - name: Desktop release candidate contract
         run: scripts/test-desktop-release-candidate.sh
       - name: OSS desktop promotion contract
diff --git a/.github/workflows/pr-image-urls.yml b/.github/workflows/pr-image-urls.yml
new file mode 100644
index 00000000000..2124361d334
--- /dev/null
+++ b/.github/workflows/pr-image-urls.yml
@@ -0,0 +1,23 @@
+name: PR Image URLs
+
+on:
+  pull_request_target:
+    types: [opened, edited, reopened]
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    name: Check PR image URLs
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    steps:
+      # pull_request_target has a privileged context. Check out only the trusted
+      # base commit and never execute code from the pull request branch.
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          ref: ${{ github.event.pull_request.base.sha }}
+          persist-credentials: false
+      - name: Check pull request body
+        run: scripts/check-pr-image-urls-event.sh
diff --git a/scripts/check-pr-image-urls-event.sh b/scripts/check-pr-image-urls-event.sh
new file mode 100755
index 00000000000..424645254bb
--- /dev/null
+++ b/scripts/check-pr-image-urls-event.sh
@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ -z "${GITHUB_EVENT_PATH:-}" ]]; then
+  echo "error: GITHUB_EVENT_PATH is required" >&2
+  exit 1
+fi
+
+script_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+markdown_file=$(mktemp)
+trap 'rm -f "$markdown_file"' EXIT
+
+if ! jq -e '(.pull_request | type) == "object"' "$GITHUB_EVENT_PATH" \
+  >/dev/null 2>&1; then
+  echo "error: event does not contain a pull_request object" >&2
+  exit 1
+fi
+
+jq -er '
+  if .pull_request.body == null then
+    ""
+  elif (.pull_request.body | type) == "string" then
+    .pull_request.body
+  else
+    error("pull_request.body must be a string or null")
+  end
+' "$GITHUB_EVENT_PATH" >"$markdown_file"
+
+"$script_dir/check-pr-image-urls.sh" "$markdown_file"
diff --git a/scripts/test-pr-image-url-event.sh b/scripts/test-pr-image-url-event.sh
new file mode 100755
index 00000000000..64470724c55
--- /dev/null
+++ b/scripts/test-pr-image-url-event.sh
@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+checker="$repo_root/scripts/check-pr-image-urls-event.sh"
+test_tmp=$(mktemp -d)
+trap 'rm -rf "$test_tmp"' EXIT
+
+bad_url="https://buzz.example.com/media/$(printf 'a%.0s' {1..64}).png"
+bad_body="Screenshot: ![broken]($bad_url)"$'\n'"\$(touch \"$test_tmp/injected\")"
+jq -n --arg body "$bad_body" \
+  '{pull_request: {body: $body}}' >"$test_tmp/bad-event.json"
+
+if GITHUB_EVENT_PATH="$test_tmp/bad-event.json" "$checker" \
+  >"$test_tmp/bad-output" 2>&1; then
+  echo "PR event checker accepted a relay media URL" >&2
+  exit 1
+fi
+grep -Fq "$bad_url" "$test_tmp/bad-output"
+grep -Fq "scripts/post-screenshots.sh" "$test_tmp/bad-output"
+[[ ! -e "$test_tmp/injected" ]]
+
+jq -n --arg body 'Screenshot: ![safe](https://github.com/user-attachments/assets/example)' \
+  '{pull_request: {body: $body}}' >"$test_tmp/good-event.json"
+GITHUB_EVENT_PATH="$test_tmp/good-event.json" "$checker"
+
+jq -n '{pull_request: {body: null}}' >"$test_tmp/empty-event.json"
+GITHUB_EVENT_PATH="$test_tmp/empty-event.json" "$checker"
+
+jq -n '{workflow_dispatch: {}}' >"$test_tmp/non-pr-event.json"
+if GITHUB_EVENT_PATH="$test_tmp/non-pr-event.json" "$checker" \
+  >"$test_tmp/non-pr-output" 2>&1; then
+  echo "PR event checker accepted an event without a pull_request object" >&2
+  exit 1
+fi
+expected_non_pr_error="error: event does not contain a pull_request object"
+if [[ "$(<"$test_tmp/non-pr-output")" != "$expected_non_pr_error" ]]; then
+  echo "PR event checker did not emit the expected non-PR event error" >&2
+  exit 1
+fi
+
+echo "PR image URL event test passed"