Join a photo hosting site like www.photobucket.com or www.flickr.com and upload your photos onto them, id reccomend flickr as its a better service, they are both free, not sure on the free limit on photo bucket is but flickr is 200, you could always upload your best onto flickr then when out of room start on photobucket. And once you done that copy and paste the image code and then when posting click on that scene pic next to the pic next to the smile drop down, when a new box comes up put the code in there and push ok
Also, make sure you are logged in a Mantidforum.Net (not .com)
It is not necessary to upload your files to another website, though some people find that easier. Seems like an extra step to me since you could just attach the file and upload it here.