I should have explained that further (this is opionion). Letting the browser compress your image defeats the purpose of thumnails. They are supposed to be quick to download so the user gets to choose whether they want to see more. The file size will remain the same if you alter the dimensions.
Also image quality is much diminshed when using browsers to resize an image, this of course will not effect a transparent spacer, but for a quality photo using the browswer just seems lazy. The compression algorythym used by browsers are simple at best. An image editor will allow much greater compression, less visual degredation and you can choose the level of optimisation. Also you can save this as a batch/command set it running and make the tea.