Every image that you upload will count towards your disk space limit. You can upload as many files as you like until you reach this disk space limit. Different SiteMaker packages have difference disk space limits, so check your 'Admin' –> 'Site statistics' section to see how much space you have in total, and left unused. Once you reach your limit, you will not be able to upload any further files, and will have to delete some in order to free up space. Or upgrade to a package with a higher disk space limit.
Some image types can be compressed to make them much smaller, even though the visual quality of the image is barely changed. JPEGs are good for this. Make sure that when you upload your image, you make it a good physical size (e.g. how big it should appear on the screen), and compress it around 70%, which will reduce the file size.
If you are uploading SWFs, please ensure that they conform to our requirements (e.g. frame rate, relative paths, export version etc) otherwise they may not load properly, may not work correctly and could effect access to your site.