diff --git a/api4/image.go b/api4/image.go
index c1087f5e2b..9022724b2a 100644
--- a/api4/image.go
+++ b/api4/image.go
@@ -20,12 +20,27 @@ func getImage(c *Context, w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		c.Err = model.NewAppError("getImage", "api.image.get.app_error", nil, err.Error(), http.StatusBadRequest)
 		return
+	} else if parsedURL.Opaque != "" {
+		c.Err = model.NewAppError("getImage", "api.image.get.app_error", nil, "", http.StatusBadRequest)
+		return
+	}
+	siteURL, err := url.Parse(*c.App.Config().ServiceSettings.SiteURL)
+	if err != nil {
+		c.Err = model.NewAppError("getImage", "model.config.is_valid.site_url.app_error", nil, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	if parsedURL.Scheme == "" {
+		parsedURL.Scheme = siteURL.Scheme
+	}
+	if parsedURL.Host == "" {
+		parsedURL.Host = siteURL.Host
 	}
 
 	// in case image proxy is enabled and we are fetching a remote image (NOT static or served by plugins), pass request to proxy
-	if *c.App.Config().ImageProxySettings.Enable && parsedURL.IsAbs() {
-		c.App.ImageProxy().GetImage(w, r, actualURL)
+	if *c.App.Config().ImageProxySettings.Enable && parsedURL.Host != siteURL.Host {
+		c.App.ImageProxy().GetImage(w, r, parsedURL.String())
 	} else {
-		http.Redirect(w, r, actualURL, http.StatusFound)
+		http.Redirect(w, r, parsedURL.String(), http.StatusFound)
 	}
 }
diff --git a/api4/image_test.go b/api4/image_test.go
index 562a4bd4f9..92bfa2d130 100644
--- a/api4/image_test.go
+++ b/api4/image_test.go
@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -100,5 +101,26 @@ func TestGetImage(t *testing.T) {
 		resp, err = th.Client.HttpClient.Do(r)
 		require.NoError(t, err)
 		assert.Equal(t, http.StatusFound, resp.StatusCode)
+
+		// protocol relative URLs should be handled by proxy
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			cfg.ServiceSettings.SiteURL = model.NewString("http://foo.com")
+		})
+		r, err = http.NewRequest("GET", th.Client.ApiUrl+"/image?url="+strings.TrimPrefix(imageServer.URL, "http:")+"/image.png", nil)
+		require.NoError(t, err)
+		r.Header.Set(model.HEADER_AUTH, th.Client.AuthType+" "+th.Client.AuthToken)
+
+		resp, err = th.Client.HttpClient.Do(r)
+		require.NoError(t, err)
+		assert.Equal(t, http.StatusOK, resp.StatusCode)
+
+		// opaque URLs are not supported, should return an error
+		r, err = http.NewRequest("GET", th.Client.ApiUrl+"/image?url=mailto:test@example.com", nil)
+		require.NoError(t, err)
+		r.Header.Set(model.HEADER_AUTH, th.Client.AuthType+" "+th.Client.AuthToken)
+
+		resp, err = th.Client.HttpClient.Do(r)
+		require.NoError(t, err)
+		assert.Equal(t, http.StatusBadRequest, resp.StatusCode)
 	})
 }