diff --git a/src/images/convert.ts b/src/images/convert.ts
new file mode 100644
index 00000000..f869a0eb
--- /dev/null
+++ b/src/images/convert.ts
@@ -0,0 +1,73 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { spawn } from 'child_process';
+import { mkdtemp } from 'fs/promises';
+import path from 'path';
+import os from 'os';
+
+let tempFolder: string;
+
+const ensureTempFolder = async() => {
+    if (!tempFolder) {
+        tempFolder = await mkdtemp(path.join(os.tmpdir(), 'slack-bridge-convert'));
+    }
+    return tempFolder;
+};
+
+// eslint-disable-next-line @typescript-eslint/promise-function-async
+const spawnConvertProcess = (inputPath: string, outputPath: string) =>
+    new Promise<void>((resolve, reject) => {
+        const ls = spawn(
+            'convert',
+            [inputPath, outputPath],
+        );
+        ls.on('close', (code) => {
+            if (code !== 0) {
+                reject(`convert exited with code ${code}`);
+            }
+            resolve();
+        });
+        ls.on('error', () => {
+            reject('Failed to start convert. Is it installed and in the PATH?');
+        });
+    });
+
+/**
+ * Converts an image from one file format to another.
+ * @param inputPath The path of the image file
+ * @param targetMimetypes A list of target mimetypes
+ */
+const convertImage = async (inputPath: string, targetMimetypes = ['image/jpeg']): Promise<string|undefined> => {
+    let outputFilename;
+    for (const mimetype of targetMimetypes) {
+        if (/^image\/jpe?g$/.test(mimetype)) {
+            outputFilename = path.basename(inputPath) + '.jpg';
+            break;
+        } else if (mimetype === 'image/png') {
+            outputFilename = path.basename(inputPath) + '.png';
+            break;
+        }
+    }
+    if (outputFilename) {
+        const folder = await ensureTempFolder();
+        const outputPath = path.join(folder, outputFilename);
+        await spawnConvertProcess(inputPath, outputPath);
+        return outputPath;
+    }
+};
+
+export default convertImage;