The dimensions of the thumbnails in your gallery pages are determined by Site Solution and the site design and layout you chose. You can't change the size of the thumbnails by dragging the corners of the image, as you would resize an image you added with the editing toolbar.
Thumbnail images in a Grid gallery appear at the same size in all designs and layouts.
The images in a Comparison page gallery, however, will adjust size in different designs and layouts to ensure that the images in each comparison pair remain adjacent to each other. If you think your before and after pictures look too big or too small, try a different layout.
An image pair in the Comparison gallery
Note: Depending on the dimensions of your own image, you may notice that your image appears cropped in its thumbnail form. Don't worry — when you click the image on your site, the full picture will appear in the gallery viewer.