You can't directly upload things to Furtopia. You have to upload them to an alternate image hosting site and then link to them in your posts. Here is how to do it.
1) Make an account on an image hosting site such as Photobucket or DeviantArt (FurAffinity will NOT work for this)
2) Upload your image to that site
3) Once you are able to view the image on the site, right click on the image and select "Copy Image Location"
4) Paste it between two img tags on Furtopia. You can type them in like the below code, or press the button above the smiley emoticon (this button ->
) and then paste in your image link.
[img]http://yourwebsite.com/.../your-pic.jpg[/img]
That's it!