make image event bodies display as blocks to avoid auto sizing, so we can measure their width to explicitly set their height

src/skins/vector/css/matrix-react-sdk/views/messages/MImageBody.css

@@ -14,6 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+.mx_MImageBody {
+    display: block;
+}
+
 .mx_MImageBody_thumbnail {
     max-width: 100%;
 /*