The button to embed your images from the gallery is not available right now since that was a custom setup. When time allows, it may come back into play.
Until then, upload your image to the gallery, go to the image, click on "Share Image". A list of options will appear. Copy the BBCode entry. And paste it in your post.
You could copy the Image Link entry and paste that in the Image URL box too, but no need really.
Another option is to upload your link to another site and insert its URL in the Image URL box.
The picture size restriction should not be different on the new system.