copy paste images into a browser

I run a blog and I usually copy/paste screenshots or gifs from my computer to the website, but lately the only thing that it does is paste the icon for the Preview application, iphoto is no help either. I've tried copy/pasting random images and gifs from google and it works so I know it's not the website. I've even tried opening my images with browsers and copy/pasted that way and it still doesn't work. I don't know what else to try I really don't want to have to download third party applications for something that used to work so well, but it's at the point now where I've passed frustrated and am just plain angry with it. I'm buying a new computer after apple's oct. 22 event and this is seriously dampening my enthusiasm for another mac computer since blogging is what I do most.

-mid 2011 iMac

-OS X Lion 10.7.5

-main browser is firefox, but I have chrome (and of course safari)

-blog on tumblr.com

-this has been a problem for maybe 2 months