Good point: it works easily
Bad point: no alt= tag, so won't validate
Thumbnail spacing can be changed by amending a .css file. Adding the line
.slimbox img { padding: 5px; }
worked for me.
If, like me, you don't work on Joomla often, you can find the .css file by following extensions>template manager
then selecting the appropriate template and clicking "edit".
Thank you for contributing this Amy.