Magento – Magento 2.3 can’t upload product images

This is caused by a bug in magento 2.3.0, which will be fixed in 2.3.1 release. You obviously cannot wait until then and you definitely don't want to edit core files.

ref: https://github.com/magento/magento2/issues/18688

If you use composer to install/update magento 2.3.0 (if you don't, you should), you can apply a patch to the code via composer.

The process is explained here:

https://support.magento.com/hc/en-us/articles/360005484154-Create-a-patch-for-a-Magento-2-Composer-installation-from-a-GitHub-commit

For completeness, here is the patch that is needed (edited as noted in the before mentioned link)

diff --git a/view/adminhtml/ui_component/design_config_form.xml b/view/adminhtml/ui_component/design_config_form.xml
index 8d4580f90c7b..bc1f36222dd6 100644
--- a/view/adminhtml/ui_component/design_config_form.xml
+++ b/view/adminhtml/ui_component/design_config_form.xml
@@ -54,7 +54,7 @@
                 <collapsible>true</collapsible>
                 <label translate="true">HTML Head</label>
             </settings>
-            <field name="head_shortcut_icon" formElement="fileUploader">
+            <field name="head_shortcut_icon" formElement="imageUploader">
                 <settings>
                     <notice translate="true">Not all browsers support all these formats!</notice>
                     <label translate="true">Favicon Icon</label>
@@ -151,7 +151,7 @@
                 <collapsible>true</collapsible>
                 <label translate="true">Header</label>
             </settings>
-            <field name="header_logo_src" formElement="fileUploader">
+            <field name="header_logo_src" formElement="imageUploader">
                 <settings>
                     <label translate="true">Logo Image</label>
                     <componentType>imageUploader</componentType>

You want to place this in a folder <magento root>/patches/composer (I named it : image-upload.diff)

and then adjust coposer.json accordingly

"extra": {
        "magento-force": "override",
        "composer-exit-on-patch-failure": true,
        "patches": {
            "magento/module-theme": {
                "MAGETWO-18688: Header logo image upload fix": "patches/composer/image-upload.diff"
            }
        }
    }

The will ensure that the fix is not wiped when you deploy/update etc

Seem like this is the magento bug in 2.3. you can check this https://github.com/magento/magento2/issues/19872

You need to override this file: /vendor/magento/module-theme/view/adminhtml/ui_component/design_config_form.xml

Change the formElement i.e Replace fileUploader with imageUploader.

I hope this will help