diff --git a/public/scripts/network.js b/public/scripts/network.js
index 91284429..0c479cc5 100644
--- a/public/scripts/network.js
+++ b/public/scripts/network.js
@@ -332,6 +332,8 @@ class Peer {
         this._filesQueue = [];
         this._busy = false;
 
+        this.maxMessageSize = 65536; // 64 KB
+
         // evaluate auto accept
         this._evaluateAutoAccept();
     }
@@ -450,14 +452,7 @@ class Peer {
 
         Events.fire('set-progress', {peerId: this._peerId, progress: 0.8, status: 'prepare'})
 
-        let dataUrl = '';
-        if (files[0].type.split('/')[0] === 'image') {
-            try {
-                dataUrl = await getThumbnailAsDataUrl(files[0], 400, null, 0.9);
-            } catch (e) {
-                console.error(e);
-            }
-        }
+        let dataUrl = await this.getFileTransferThumbnail(files[0]);
 
         Events.fire('set-progress', {peerId: this._peerId, progress: 1, status: 'prepare'})
 
@@ -472,6 +467,27 @@ class Peer {
         Events.fire('set-progress', {peerId: this._peerId, progress: 0, status: 'wait'})
     }
 
+    async getFileTransferThumbnail(image) {
+        if (image.type.split('/')[0] !== 'image') return '';
+
+        let dataUrl = '';
+        try {
+            // Iteratively lower thumbnail quality until its size is less than maxMessageSize - 2 kB
+            let quality = 1;
+            do {
+                quality -= 0.1;
+                if (quality <= 0) {
+                    console.error("Could not create thumbnail that fits into one message.");
+                    return '';
+                }
+                dataUrl = await getThumbnailAsDataUrl(image, 450, 450, quality);
+            } while (new Blob([dataUrl]).size + 2_000 > this.maxMessageSize);
+        } catch (e) {
+            console.error(e);
+        }
+        return dataUrl;
+    }
+
     async sendFiles() {
         for (let i=0; i<this._filesRequested.length; i++) {
             this._filesQueue.push(this._filesRequested[i]);
@@ -725,7 +741,8 @@ class RTCPeer extends Peer {
         super(serverConnection, isCaller, peerId, roomType, roomId);
 
         this.rtcSupported = true;
-        this.rtcConfig = rtcConfig
+        this.rtcConfig = rtcConfig;
+        this.maxMessageSize = 262144; // 256 KB
 
         if (!this._isCaller) return; // we will listen for a caller
         this._connect();
@@ -811,6 +828,13 @@ class RTCPeer extends Peer {
         Events.on('beforeunload', e => this._onBeforeUnload(e));
         Events.on('pagehide', _ => this._onPageHide());
         Events.fire('peer-connected', {peerId: this._peerId, connectionHash: this.getConnectionHash()});
+        this._setMaxMessageSize();
+    }
+
+    _setMaxMessageSize() {
+        this.maxMessageSize = this._conn && this._conn.sctp
+            ? Math.min(this._conn.sctp.maxMessageSize, 1048576) // 1 MB max
+            : 262144; // 256 KB
     }
 
     _onMessage(message) {
diff --git a/public/scripts/ui.js b/public/scripts/ui.js
index c9bd3658..1d7761af 100644
--- a/public/scripts/ui.js
+++ b/public/scripts/ui.js
@@ -281,10 +281,8 @@ class PeersUI {
 
             if (files[0].type.split('/')[0] === 'image') {
                 try {
-                    let imageUrl = await getThumbnailAsDataUrl(files[0], 80, null, 0.9);
-
+                    let imageUrl = await getThumbnailAsDataUrl(files[0], 80, 80, 0.9);
                     this.$shareModeImageThumb.style.backgroundImage = `url(${imageUrl})`;
-
                     this.$shareModeImageThumb.removeAttribute('hidden');
                 } catch (e) {
                     console.error(e);
diff --git a/public/scripts/util.js b/public/scripts/util.js
index fc418ff8..36d212ca 100644
--- a/public/scripts/util.js
+++ b/public/scripts/util.js
@@ -493,26 +493,40 @@ function getThumbnailAsDataUrl(file, width = undefined, height = undefined, qual
 
             await waitUntilImageIsLoaded(imageUrl);
 
-            let imageWidth = image.width;
-            let imageHeight = image.height;
             let canvas = document.createElement('canvas');
+            let heightForSpecifiedWidth;
+            let widthForSpecifiedHeight;
 
-            // resize the canvas and draw the image data into it
+            if (width) {
+                heightForSpecifiedWidth = Math.floor(image.height * width / image.width);
+            }
+            if (height) {
+                widthForSpecifiedHeight = Math.floor(image.width * height / image.height);
+            }
+
+            // resize the canvas and draw the image on it
             if (width && height) {
-                canvas.width = width;
-                canvas.height = height;
+                // mode "contain": preserve aspect ratio and use arguments as boundaries
+                if (height > heightForSpecifiedWidth) {
+                    canvas.width = width;
+                    canvas.height = heightForSpecifiedWidth;
+                }
+                else {
+                    canvas.width = widthForSpecifiedHeight;
+                    canvas.height = height;
+                }
             }
             else if (width) {
                 canvas.width = width;
-                canvas.height = Math.floor(imageHeight * width / imageWidth)
+                canvas.height = heightForSpecifiedWidth;
             }
             else if (height) {
-                canvas.width = Math.floor(imageWidth * height / imageHeight);
+                canvas.width = widthForSpecifiedHeight;
                 canvas.height = height;
             }
             else {
-                canvas.width = imageWidth;
-                canvas.height = imageHeight
+                canvas.width = image.width;
+                canvas.height = image.height
             }
 
             let ctx = canvas.getContext("2d");