Blurred Slider image

A few months ago I created a couple of websites, for different clients using 'Adventurous'. I used the correct dimensions for the slider images and the slider images looked lovely and clear on my laptop, iPad and iPhone. This week I noticed that the slider imaged on the iPad suddenly looked blurred. This has happened on both websites. I also checked on another site I had created last year using 'Catch Everest' and this too has blurry slider images now. What can be done to rectify this, I was about to make a premium purchase from you for an e-commerce site, but need to ensure that the slider images look good on all devices?