You can pretty much drag everything at Wrike, which is great.
However not being able to drag an uploaded image from attachments to description area is very counter intuitive.
Even if you decide not to implement this feature, I believe you have to improve current workflow. Because current steps are:
1- click image icon on toolbar
2- click insert from uploads
3- select the one you like to add description
4- click insert
5- click the arrow on the image
6- click expand
You can easily eliminate items 1-4 by just enabling dragging feature.