When adding a portrait-formatted image and re-sizing that image to 300px the image gets blurry. Landscape-format images are not affected, but the narrower the aspect ratio in portrait, the blurrier they appear in the 300px thumbnail size.

Upon closer inspection. You will see that the Confluence is trying to resize a thumbnail

src="/download/thumbnails/753668/2013-10-14%2009.03.34.png?version=1&modificationDate=1381934123961&api=v2"

instead of the source

src="/download/attachments/753668/2013-10-14%2009.03.34.png?version=1&modificationDate=1381934123961&api=v2"