The colorbox and insert modules, as mentioned already, would solve your problem. First you want to create different image styles based on the sizes you would like to have, if they are standard. Then, the insert module would allow you to place those anywhere in the text of that specified style, one of which, for example, would be 'Colorbox: thumbnail.' When you click it, it would show up in a lightbox.
Another way to do this, which is the way I do sometimes, is using CKEditor with CKFile which will allow you to manually upload, resize, and insert any image at anysize you would like anywhere in your content.
Then, I use lightbox2 module and wrap that image in an A tag, href=# with rel="lightbox2"
with lightbox2 installed, this will force that tag to be opened in a lightbox. Be sure to read the lightbox2 help file as it will explain thorougly how to configure your A tags based on what you want to do with the images in your post.