Most likely you'll need to downsize the image.
Most scanners and camera take very high resolution images; that means that, on a computer monitor, the 100% zoomed in image is a lot bigger than the actual picture. This is so it prints at high quality. However, it also makes the files themselves very large; so in order to post them online you'll need to shrink them down a bit.
I know Gimp can size images down, and I'm pretty sure MS Paint can. Alternatively, if you upload directly to photobucket, there's an option you can select so that it will size them down for you when it uploads.
Just bear in mind that images posted directly on the forums (meaning they show up without you having to click a link) are limited to 800 pixels in width; so if it's any larger than that it'll need to be further downsized or posted as a link.