You know. I did. Its actually really easy.
Get rid of this plugin.
Then upload your slider featured image into the post. Update.
(WordPress creates those sizes, images that you see as the “featured” in the post lists when you upload that image.)
Just go to the image on the blog list.
Right click and click on get the image URL (what it is called and the path to where it is.)
It should look something like this:
https://ursite.com/wp-content/uploads/2012/08/urfilename-150×150.gif
Just download an FTP client (FileZilla works for me).
Save the image you would like at that size (example is 150×150 px) and exactly that name (end of URL, italicized above)
FTP it to your site
