I discovered a hacky workaround that allows me to paste images into Bear 2 beta as thumbnails!
Bear 2 adds HTML comment text like <!-- {"width":255} --> immediately following images to define their size. You can discover this by adding three ticks ``` directly before an image you’ve resized to expose the underlying markdown.
It turns out that pasting text like this immediately after an image will resize the image. So, I adapted my Alfred script that copies & pastes a screenshot from my secondary monitor during presentations to type <!-- {"width":255} --> into the keyboard immediately after pasting the image and – voilà! – my pasted images are converted to a 255px-wide thumbnail! I no longer have to live with full-width images in my Bear notes! Yay! 
Now I’ll make another script (or hotkey or BetterTouchTool gesture) to paste this same text so I can instantly convert images to my preferred thumbnail size simply by positioning the cursor after an image and triggering the script. 
Another benefit, is you can put text like <!-- {"width":255} --> into your clipboard (change 255 to your preferred thumbnail width) and go through any notes migrated to Bear 2 with gigantic (full width) images and fairly quickly convert them to thumbnails simply by moving the cursor immediately after each image and pasting the “magic” thumbnail-sizing text. Just don’t forget ⌘Z is an easy way to revert a change anytime you accidentally paste in the wrong place (which will happen occasionally if your doing this for lots of images).
As a side note, it looks like pasting text like <!-- {"width":255} --> after an image that has already been resized results in two (or more) of these HTMl comments. Fortunately, Bear 2 uses the last one, so you can resize images manually after doing this trick (and continue to do this trick after manually resizing); however, the existence of multiple HTML comments might cause a problem down the road. So, I’m still hopeful Bear 2 will eventually add an option to paste as thumbnail by default and I won’t have to continue using this hacky workaround.