This is possible by adding the following into your “custom CSS” box in the settings (pro only I think).
/* image - sizing /
/ Images are shrunk to 50 pixel thumbnails /
.node-displayed-image { max-width: 100px; max-height: 70px; }
/ Hovering over an image thumbnail enbiggens it */
.node-displayed-image:hover { max-width: 100%; max-height: 100%; }
Hovering pops up a larger image - clicking it opens a new tab with the image in. I find it extremely useful.
I copied the code from George3 who had kindly posted it here:
You can change the numbers to change the size to your preference.