Depending on the source of your images, you might be using over-size images and can reduce load times by creating smaller resolution versions for the web site. This would be the first thing you do. I say separate versions because you probably don’t want to downscale (and lose) your master images lest you change them later.
When you look at an image in place in Blocs, it tells you the resolution (in the sidebar and next to the image) as nnn x mmm pixels. This is the size you need, assuming that the user views the website in similar conditions to you. If your image is larger than this then you can usefully produce a website version. If you want to allow for high resolution (retina) displays, the images need to be at 144dpi, otherwise use the standard of 72dpi; feed these numbers into your image processing software and export a version for the website.
This exported image is the one to be compressed. JPEG is generally the best format and you can use the compression setting in your image processing app’s export facility to control it and/or the tricks that @ViaWebDesign suggested.
Personally, an option in Blocs to automatically downscale an oversized image when it is imported is on my wish list for @Norm; it would not be universally applicable but would save time with images that are just eye-candy and whose quality is not a site design decision.