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I'm using the Visual Composer plugin to build, or basically visually insert the shortcode for a number of sliders, latest blog posts and masonry grids. Most of these are set to use a posts featured image as the background to the link.

I'm not sure how to set the size of these images though, so they always appear as the original featured image, and the size is just scaled down in the CSS. This is obviously flagging up a big problem in PageSpeed Insights and is a part of why my site is so hideously slow.

I appreciate that it needs to be a certain size to scale properly, but is there a way to set the size of this image that is served? I need to keep the original, larger upload as that is shown on the actual post page.