In Nautilus File Manager (called Files in the app menu), click the hamburger icon.
Select Preferences from the drop down menu.
In the Preferences Window, click the Search & Preview tab.
What you are looking for is Thumbnails.
Set your preferences - whether to always show, or just on your computer only... and set the Size limit. Let's say you want Thumbnail previews to show even on Large Files (Takes more memory), increase the size in the drop down menu uner "Only for files smaller than".
Thanks a lot. Had to dabble a bit, as my system is in French, vocabulary is of course slightly different. Works fine, but only if you choose a view by icons, not as a list. Am I right, or did I fail to understand something once more ?