LXDE: PCManFM Thumbnails of images > 2MB are not shown

By default pcmanfm limits the size of files to show thumnails to 2 Mb.

Go in the menu Edit > Preferences > Display

At the bottom, change the value of "Do not generate thumbnails for file exceeding this size" from 2000 KB to a larger size.

10000 KB (10MB) perhaps. Depends on the size of your pictures.