Online comics are often distributed in .cbr or .cbz formats, which cannot be opened with any built-in applications on Mac.
What are .cbr .cbz files
These files contain a sequence of image files (usually PNG or JPEG formats).
.cbr uses RAR compression, .cbz uses ZIP, .cb7 uses 7-Zip, .cbt uses TAR and .cba uses ACE.
If you rename them to .rar/.zip etc., you can get the images. However, doing this reduces the quality of files, so a better way is to use a specialized reader for these files.
FFViewer is a free tool for Mac (10.4 Tiger and above) to open .cbz and .cbr files.
FFViewer has an easy-to-use interface. Right click and select Open with > FF Viewer. FFViewer will then show a list of images within its file listing, and just clicking on the image window takes you to next image.