Has WordPress.com added the ability to change the default size for thumbnails? 128 pixels is smaller than ideal...something like 250 or 300 would be much better, at least for the situations I encounter.
My trick to solve this problem in the past was to upload the full-size image and simply resize it in the post. What I -didn't- realize when I first started doing that is that WordPress doesn't really resize the image...it just resizes the <img> width and height, meaning that the full image is getting downloaded every time the page is viewed! So in fact, this approach makes WordPress.com servers work 10x as hard and dramatically increases page load time.
So: if this feature doesn't exist yet, it would be phenomenally nice to have! Thanks.
(I know that I can create my own virtual "thumbnail" at the size I want, upload it, then upload my full-size screenshot somewhere else and link the two together...but that takes something that should be one step, turns it into ten steps, forces me to create multiple copies of the same image, etc. It's a lot of work for something that should be trivial.)