1. Take a photo with a digital camera, then upload the photos from your camera to your computer. Then open the photo with your whatever paint or photo-editing program you might have so you can crop it and resize it. Then upload the photo with a post, by going to the "browse" box you see at the bottom of the post page, and upload it to TreasureNet.
2. Use a scanner. For small objects such as coins and buttons, this works better, as it gives more detail in sharp focus. Then edit with your software, as above, and upload that.
A lot of printers/copiers now have scanners built in. If you don't have one stores like Kinkos will let you rend one for cheap.