I wonder what the ideal image size (resolution) for post images on our website is (www.morga.ch, Integrity Light child). We currently use images with 1200×600 px @72ppi for all posts. But the latest image i uploaded to a post (http://www.morga.ch/?page_id=12133&preview_id=12133&preview_nonce=2285eb4f2a&preview=true=, also see screenshot please) seems to be too small, since the font is displayed slightly blurred (also see screenshot attached). Here is another example for a post in case you can not open the link above: http://www.morga.ch/wettbewerb-bioladentag-2015/.
Please disable all third party plugins, clear cache and check again.
If it doesn’t help, please switch to another theme and check if the issue only happens when X is active or not.