You don't have to drag and drop the image onto the bone. You can select the image, then click Set Parent (or press P), then click the bone in the viewport (or tree).
You can also use page up/down to navigate your selection history, which can help in many cases. For example, drag and drop the first image to a bone, by scrolling up to find the bone. Now the bone is selected, but you want to grab another image, so hit page down to go back to the image you had selected. Start dragging a new image (or use Set Parent) and now you want a bone, press page up to select the bone you had selected, which is probably near the bone you want, then drop it on a bone. While dragging you can use right click to expand nodes to find the bone you want. Right click on the root bone or double click the Expand button to expand all nodes. Also you can filter the tree to show only bones to cut down on the noise as you try to find a bone.
Another way to do it is to use Import Data to bring new images into Spine already in the correct position, then you don't have to create attachments and place them manually at all, which can save a lot of time. Here's a quick clip showing it:
https://www.youtube.com/watch?v=kU26B5465SQ&t=406
