You need to keep the images, and ID must be able to find them. Broken or out of date links will result in poor quality output. All that is saved inside the file is a low-res screen preview.
If all that's saved in the file is a low-resolution preview, how come my file is 80 MB? So far I have about 30 pages with pictures on them.
If i keep up at this rate, I will soon be running out of disk space .
…how come my file is 80 MB?
Maybe some combination of a few factors. First, you can usually make the file size smaller if you periodically do a save-as. InDesign saves some data that deals with undos, and doing a save-as will wipe clean some info that is probably not doing anything for you. Also, if you are saving your images with 72ppi resolution, but shrinking the images after they are placed into InDesign to achieve a higher effective resolution, consider setting the target resolution to the file in Photoshop with resampling turned off. The image will look the same, you won't have to make them smaller after you place them, and the preview portion of the image (the part that gets saved in the InDesign file) will be smaller. You can try it with a test file if you like. Just place 5 or 6 72ppi images on a page with 300ppi effective resolution and save the file. Next, change the images to 300ppi in Photoshop and place them into a new doc and compare the file size. It will likely be lower.
Maybe that you have placed the images with embedding them and not linking them. This will increase the files size.
But this would not be a recommended workflow because it is often causing problems and eventually ends in file corruption. Better is to link images, and go to File > Package… and keep that folder for the future.