No Thumbnail for an Uploaded Image in Wordpress
A Short Answer to a Specific Question:
If you upload an image in Wordpress, and you don’t get the option to “Show Thumbnail” in the “Send to Editor”, it’s probably because the image was too big.
Also: there is no good way to resize an image in iPhoto.
A free tool to resize images on the Mac is ImageWell, from XtraLean Software.