Overlay added for inserting images through orgitdown editor.

Previously "Picture" button on editor insert a default block of code in the editor which specifies Image  caption , width & Image Url.

Instead of this now overlay provided where user has to provide above fields ( Name, Width, Url ) and after adding this image the following block of code will be inserted in the editor:
eg:
##+CAPTION: Image Name
#+ATTR_HTML:  Image Width
[[Image URL]]
 

Please test the following after taking this pull:
1. Go to edit view of any resource or page where orgitdown editor is available.
2. Click on "Picture" button.
3. Overlay with three inputs will get visible which ask for "Image Name", "Width" (image width in pixel) & "URL" (image absolute url).
4. Click "Add Image".