Glad to see this still in development.
I believe there is an issue with loading PNG files. When I try to load them (either through drag and drop or by the open button) Laigter will show the texture is loaded, but nothing appears in the preview window.
Here is an attachment of the window (right side only with relevant info):
Basically, any image (with transparency) does not show up in Laigter even though the Textures window indicates otherwise.
NOTE: This was a bug in 1.4 also, but I couldn't find the time to report it back then.