As some of you might have already noticed, we have made updates to the preview images on WinCustomize. There was a couple of reasons this was changed, and one of those is for in-app browsing of skins.
With these changes made, and that we have several apps already supporting in-app browsing, we are asking all skinners and uploaders to now upload preview images in a 16:9 format.
We are specifically recommending (and this has been noted on the upload page) a 1920x1080 resolution for your preview images.
Right now we want to get the word out about the changes, but in the coming weeks this will become more of a requirement to ensure a consistent preview experience.
Thanks for your support and for all the wonderful creations you share with the community!